Make the askappend() prompt non-modal.

This fits into a new dialog-box slot (because it might have to come up
at the same time as a network prompt), and makes use of the existing
callback system in logging.c which buffers the logging data until the
user says what they want done with it.

unix/unix.h

@@ -152,6 +152,7 @@ GtkWidget *get_window(void *frontend);
 enum DialogSlot {
     DIALOG_SLOT_RECONFIGURE,
     DIALOG_SLOT_NETWORK_PROMPT,
+    DIALOG_SLOT_LOGFILE_PROMPT,
     DIALOG_SLOT_LIMIT /* must remain last */
 };
 void register_dialog(void *frontend, enum DialogSlot slot, GtkWidget *dialog);