Add a configuration option for fullscreen locking

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

config.def.h

@@ -35,6 +35,7 @@ static const Rule rules[] = {
 static const float mfact     = 0.55; /* factor of master area size [0.05..0.95] */
 static const int nmaster     = 1;    /* number of clients in master area */
 static const int resizehints = 1;    /* 1 means respect size hints in tiled resizals */
+static const int lockfullscreen = 1; /* 1 will force focus on the fullscreen window */
 
 static const Layout layouts[] = {
 	/* symbol     arrange function */

dwm.c

@@ -835,7 +835,7 @@ focusstack(const Arg *arg)
 {
 	Client *c = NULL, *i;
 
-	if (!selmon->sel || selmon->sel->isfullscreen)
+	if (!selmon->sel || (selmon->sel->isfullscreen && lockfullscreen))
 		return;
 	if (arg->i > 0) {
 		for (c = selmon->sel->next; c && !ISVISIBLE(c); c = c->next);