[dwm] bug in grabkeys()

Hi,

Changeset 1320:65c37b4be9cb introduces a bug in grabkeys() that causes
MODKEY presses to be ignored if numlock is enabled on my system.

The problem is that between initializing modifiers[] with numlockmask
and using it later, numlockmask is changed without updating modifiers[].

Maybe extract a function initmodmap(), to be called before grabkeys(),
from the modmap initializiation block? Right now grabbuttons(), which
uses numlockmask as well, subtly depends on grabkeys() having been
called beforehand; this might make it a little more explicit what's
going on.

I've attached a patch that works for me, hopefully someone who knows
more about that stuff can look over it.

Regards,
Peter