Add the ability to close sessions. This adds *_free() functions to most

areas of the code.  Not all back-ends have been tested, but Telnet and SSH
behave reasonably.

Incidentally, almost all of this patch was written through Mac PuTTY,
admittedly over a Telnet connection.

[originally from svn r2615]

logging.c

@@ -170,6 +170,14 @@ void *log_init(void *frontend, Config *cfg)
     return ctx;
 }
 
+void log_free(void *handle)
+{
+    struct LogContext *ctx = (struct LogContext *)handle;
+
+    logfclose(ctx);
+    sfree(ctx);
+}
+
 void log_reconfig(void *handle, Config *cfg)
 {
     struct LogContext *ctx = (struct LogContext *)handle;