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

@@ -1,6 +1,7 @@
 /* appearance */
 static const int sloppyfocus        = 1;  /* focus follows mouse */
 static const unsigned int borderpx  = 1;  /* border pixel of windows */
+static const int lockfullscreen     = 1;  /* 1 will force focus on the fullscreen window */
 static const float rootcolor[]      = {0.3, 0.3, 0.3, 1.0};
 static const float bordercolor[]    = {0.5, 0.5, 0.5, 1.0};
 static const float focuscolor[]     = {1.0, 0.0, 0.0, 1.0};

dwl.c

@@ -1177,7 +1177,7 @@ focusstack(const Arg *arg)
 {
 	/* Focus the next or previous client (in tiling order) on selmon */
 	Client *c, *sel = selclient();
-	if (!sel)
+	if (!sel || (sel->isfullscreen && lockfullscreen))
 		return;
 	if (arg->i > 0) {
 		wl_list_for_each(c, &sel->link, link) {