Add --no-ewmh-fullscreen

This arg reverts to the old behaviour of checking for fullscreen
windows.

src/config.c

@@ -554,6 +554,7 @@ char *parse_config(options_t *opt, const char *config_file, bool *shadow_enable,
 	    .focus_blacklist = NULL,
 	    .detect_transient = false,
 	    .detect_client_leader = false,
+	    .no_ewmh_fullscreen = false,
 
 	    .track_wdata = false,
 	    .track_leader = false,