Do not allow focus to drift from fullscreen client via focusstack()

It generally doesn't make much sense to allow focusstack() to navigate
away from the selected fullscreen client, as you can't even see which
client you're selecting behind it.

I have had this up for a while on the wiki as a separate patch[0], but
it seems reasonable to avoid this behaviour in dwm mainline, since I'm
struggling to think of any reason to navigate away from a fullscreen
client other than a mistake.

0: https://dwm.suckless.org/patches/alwaysfullscreen/