GTK 3 prep: write a replacement for gtk_quit_add().

GTK 2 has deprecated it and provided no replacement; a bug tracker
entry I found on the subject suggested that it was functionality that
didn't really belong in GTK, and glib ought to provide a replacement
instead, which would be a perfectly fine thing to suggest if they had
waited for glib to get round to doing so *before* throwing out a
function people were actually using. Sigh.

Anyway, it turns out that subsidiary invocations of gtk_main() don't
happen inside GTK as far as I can see, so all I need to do is to make
sure my own invocations of gtk_main() are followed by a cleanup
function which runs any quit functions that I've registered.

That was the last deprecated GTK function, so we now build cleanly
with -DGTK_DISABLE_DEPRECATED. (But, as mentioned a couple of commits
ago, we still don't build with -DGDK_DISABLE_DEPRECATED, because that
has migrating to Cairo drawing as a prerequisite.)

unix/unix.h

@@ -80,6 +80,7 @@ long get_windowid(void *frontend);
 
 /* Things gtkdlg.c needs from pterm.c */
 void *get_window(void *frontend);      /* void * to avoid depending on gtk.h */
+void post_main(void);     /* called after any subsidiary gtk_main() */
 
 /* Things pterm.c needs from gtkdlg.c */
 int do_config_box(const char *title, Conf *conf,