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

config.def.h

@@ -567,6 +567,7 @@ static const int nmaster     = 1;    /* number of clients in master area */
 static const int nstack      = 0;    /* number of clients in primary stack area */
 #endif // FLEXTILE_DELUXE_LAYOUT
 static const int resizehints = 0;    /* 1 means respect size hints in tiled resizals */
+static const int lockfullscreen = 1; /* 1 will force focus on the fullscreen window */
 #if DECORATION_HINTS_PATCH
 static const int decorhints  = 1;    /* 1 means respect decoration hints */
 #endif // DECORATION_HINTS_PATCH