[dwm] grabbuttons

Hello.

I started trying out dwm (5.0.1) yesterday, and I really like it! Good
work there.

However, I encountered some problems when I was modifying the actions
related to window mouse clicking. Changing the event mask for
ClkClientWin in buttons array did not work without modifying
dwm.c:grabbuttons() accordingly. Therefore, I removed the assumption
that MODKEY would be used as event mask, and instead grab only the
buttons and event masks that really are used; see grabbuttons.diff.
Maybe something that Anselm likes to apply?

/Fredrik