Refactor the LogContext type.

LogContext is now the owner of the logevent() function that back ends
and so forth are constantly calling. Previously, logevent was owned by
the Frontend, which would store the message into its list for the GUI
Event Log dialog (or print it to standard error, or whatever) and then
pass it _back_ to LogContext to write to the currently open log file.
Now it's the other way round: LogContext gets the message from the
back end first, writes it to its log file if it feels so inclined, and
communicates it back to the front end.

This means that lots of parts of the back end system no longer need to
have a pointer to a full-on Frontend; the only thing they needed it
for was logging, so now they just have a LogContext (which many of
them had to have anyway, e.g. for logging SSH packets or session
traffic).

LogContext itself also doesn't get a full Frontend pointer any more:
it now talks back to the front end via a little vtable of its own
called LogPolicy, which contains the method that passes Event Log
entries through, the old askappend() function that decides whether to
truncate a pre-existing log file, and an emergency function for
printing an especially prominent message if the log file can't be
created. One minor nice effect of this is that console and GUI apps
can implement that last function subtly differently, so that Unix
console apps can write it with a plain \n instead of the \r\n
(harmless but inelegant) that the old centralised implementation
generated.

One other consequence of this is that the LogContext has to be
provided to backend_init() so that it's available to backends from the
instant of creation, rather than being provided via a separate API
call a couple of function calls later, because backends have typically
started doing things that need logging (like making network
connections) before the call to backend_provide_logctx. Fortunately,
there's no case in the whole code base where we don't already have
logctx by the time we make a backend (so I don't actually remember why
I ever delayed providing one). So that shortens the backend API by one
function, which is always nice.

While I'm tidying up, I've also moved the printf-style logeventf() and
the handy logevent_and_free() into logging.c, instead of having copies
of them scattered around other places. This has also let me remove
some stub functions from a couple of outlying applications like
Pageant. Finally, I've removed the pointless "_tag" at the end of
LogContext's official struct name.

logging.c

@@ -12,12 +12,12 @@
 #include "putty.h"
 
 /* log session to file stuff ... */
-struct LogContext_tag {
+struct LogContext {
     FILE *lgfp;
     enum { L_CLOSED, L_OPENING, L_OPEN, L_ERROR } state;
     bufchain queue;
     Filename *currlogfilename;
-    Frontend *frontend;
+    LogPolicy *lp;
     Conf *conf;
     int logtype;		       /* cached out of conf */
 };
@@ -48,9 +48,8 @@ static void logwrite(LogContext *ctx, void *data, int len)
 	if (fwrite(data, 1, len, ctx->lgfp) < (size_t)len) {
 	    logfclose(ctx);
 	    ctx->state = L_ERROR;
-	    /* Log state is L_ERROR so this won't cause a loop */
-	    logevent(ctx->frontend,
-		     "Disabled writing session log due to error while writing");
+            lp_eventlog(ctx->lp, "Disabled writing session log "
+                        "due to error while writing");
 	}
     }				       /* else L_ERROR, so ignore the write */
 }
@@ -121,23 +120,14 @@ static void logfopen_callback(void *vctx, int mode)
 		       ctx->logtype == LGTYP_SSHRAW ? "SSH raw data" :
 		       "unknown"),
 		      filename_to_str(ctx->currlogfilename));
-    logevent(ctx->frontend, event);
+    lp_eventlog(ctx->lp, event);
     if (shout) {
         /*
          * If we failed to open the log file due to filesystem error
          * (as opposed to user action such as clicking Cancel in the
-         * askappend box), we should log it more prominently. We do
-         * this by sending it to the same place that stderr output
-         * from the main session goes (so, either a console tool's
-         * actual stderr, or a terminal window).
-         *
-         * Of course this is one case in which that policy won't cause
-         * it to turn up embarrassingly in a log file of real server
-         * output, because the whole point is that we haven't managed
-         * to open any such log file :-)
+         * askappend box), we should log it more prominently.
          */
-        from_backend(ctx->frontend, 1, event, strlen(event));
-        from_backend(ctx->frontend, 1, "\r\n", 2);
+        lp_logging_error(ctx->lp, event);
     }
     sfree(event);
 
@@ -188,8 +178,8 @@ void logfopen(LogContext *ctx)
 	if (logxfovr != LGXF_ASK) {
 	    mode = ((logxfovr == LGXF_OVR) ? 2 : 1);
 	} else
-	    mode = askappend(ctx->frontend, ctx->currlogfilename,
-			     logfopen_callback, ctx);
+            mode = lp_askappend(ctx->lp, ctx->currlogfilename,
+                                logfopen_callback, ctx);
     } else
 	mode = 2;		       /* create == overwrite */
 
@@ -223,16 +213,32 @@ void logtraffic(LogContext *ctx, unsigned char c, int logmode)
  * Log an Event Log entry. Used in SSH packet logging mode, to copy
  * the Event Log entries into the same log file as the packet data.
  */
-void log_eventlog(LogContext *ctx, const char *event)
+void logevent(LogContext *ctx, const char *event)
 {
-    /* If we don't have a context yet (eg winnet.c init) then skip entirely */
     if (!ctx)
-	return;
-    if (ctx->logtype != LGTYP_PACKETS &&
-	ctx->logtype != LGTYP_SSHRAW)
-	return;
-    logprintf(ctx, "Event Log: %s\r\n", event);
-    logflush(ctx);
+        return;
+    if (ctx->logtype == LGTYP_PACKETS || ctx->logtype == LGTYP_SSHRAW) {
+        logprintf(ctx, "Event Log: %s\r\n", event);
+        logflush(ctx);
+    }
+    lp_eventlog(ctx->lp, event);
+}
+
+void logevent_and_free(LogContext *ctx, char *event)
+{
+    logevent(ctx, event);
+    sfree(event);
+}
+
+void logeventf(LogContext *ctx, const char *fmt, ...)
+{
+    va_list ap;
+    char *buf;
+
+    va_start(ap, fmt);
+    buf = dupvprintf(fmt, ap);
+    va_end(ap);
+    logevent_and_free(ctx, buf);
 }
 
 /*
@@ -359,12 +365,12 @@ void log_packet(LogContext *ctx, int direction, int type,
     logflush(ctx);
 }
 
-LogContext *log_init(Frontend *frontend, Conf *conf)
+LogContext *log_init(LogPolicy *lp, Conf *conf)
 {
     LogContext *ctx = snew(LogContext);
     ctx->lgfp = NULL;
     ctx->state = L_CLOSED;
-    ctx->frontend = frontend;
+    ctx->lp = lp;
     ctx->conf = conf_copy(conf);
     ctx->logtype = conf_get_int(ctx->conf, CONF_logtype);
     ctx->currlogfilename = NULL;