reset X11's "interface has been initialized" flag

Reset 'x_inited' after the various widgets have been released
during shutdown.

This might prevent the second panic ('X11_mark_synch()' during
emergency save) in the double panic reported in a later comment of
github issue #569.  It definitely doesn't address whatever caused
the first panic, nor the poor handling of missing fonts that was
apparently responsible for #569's initial report.

win/X11/winX.c

@@ -1658,6 +1658,8 @@ X11_exit_nhwindows(const char *dummy)
 
     release_getline_widgets();
     release_yn_widgets();
+
+    x_inited = FALSE;
 }
 
 #ifdef X11_HANGUP_SIGNAL