Add a configuration option for fullscreen locking

Some people are annoyed to have this new behaviour forced for some
application which use fake fullscreen.

Ref. https://git.suckless.org/dwm/commit/138b405f0c8aa24d8a040cc1a1cf6e3eb5a0ebc7.html

dwm.c

@@ -2018,7 +2018,7 @@ focusstack(const Arg *arg)
 	if (!selmon->sel || (selmon->sel->isfullscreen && !selmon->sel->fakefullscreen))
 		return;
 	#else
-	if (!selmon->sel || selmon->sel->isfullscreen)
+	if (!selmon->sel || (selmon->sel->isfullscreen && lockfullscreen))
 		return;
 	#endif // LOSEFULLSCREEN_PATCH
 	#if BAR_WINTITLEACTIONS_PATCH