Add asynchronous callback capability to the askappend() alert box.

This was harder than verify_ssh_host_key() and askalg() put
together, because:
 (a) askappend() can be called at any time, since it's a side effect
     of data-logging functions. Therefore there can be an unfinished
     askappend() alert at any time, and hence the OS X front end has
     to be prepared to _queue_ other alerts which occur during that
     time.
 (b) logging.c has to do something with data that comes in while
     it's waiting for an answer to askappend(). It buffers it until
     it knows what the user wants done with it. This involved
     something of a reorganisation of logging.c.

[originally from svn r5344]

unix/gtkdlg.c

@@ -2753,7 +2753,8 @@ void logevent_dlg(void *estuff, const char *string)
     es->nevents++;
 }
 
-int askappend(void *frontend, Filename filename)
+int askappend(void *frontend, Filename filename,
+	      void (*callback)(void *ctx, int result), void *ctx)
 {
     static const char msgtemplate[] =
 	"The session log file \"%.*s\" already exists. "

unix/uxcons.c

@@ -198,7 +198,8 @@ int askalg(void *frontend, const char *algtype, const char *algname,
  * Ask whether to wipe a session log file before writing to it.
  * Returns 2 for wipe, 1 for append, 0 for cancel (don't log).
  */
-int askappend(void *frontend, Filename filename)
+int askappend(void *frontend, Filename filename,
+	      void (*callback)(void *ctx, int result), void *ctx)
 {
     static const char msgtemplate[] =
 	"The session log file \"%.*s\" already exists.\n"