Reimplement 'really close session?' as a non-modal message box.

I've also moved it out into gtkwin.c, because it seemed easier to do
the 'find existing instance of this dialog and raise it' dance there
than to split it across source files pointlessly.

unix/gtkdlg.c

@@ -3467,17 +3467,6 @@ int message_box(
     return retval;
 }
 
-int reallyclose(void *frontend)
-{
-    char *title = dupcat(appname, " Exit Confirmation", NULL);
-    int ret = message_box(GTK_WIDGET(get_window(frontend)), title,
-                          "Are you sure you want to close this session?",
-                          string_width("Most of the width of the above text"),
-                          FALSE, &buttons_yn);
-    sfree(title);
-    return ret;
-}
-
 struct verify_ssh_host_key_result_ctx {
     char *host;
     int port;