Ignore re-focusing the focused client

Ignore re-focusing the focused client

When something gives the input focus to the client which already has the input
focus, bad things can happen. Normally, you'd expect nothing to happen in this
case, but X11 is not that simple.

When updating the input focus and the focused client has the nofocus hint set,
we are actually taking away the focus from this client.

Hopefully this fixes FS#973.

Signed-off-by: Uli Schlachter <psychon@znc.in>