[dwm] dmenu filename tab-completion patch

I always try to do this and get frustrated when I remember it doesn't
work, but today I actually looked up the man pages and found out how
easy it is to get dmenu to support filename tab-completion. This patch
adds a small function that completes a filename argument as far as
possible. If you'd rather complete to the first match, just comment
or delete the block specified. With this patch it is now possible to
type 'ooc<tab> ~/big<tab>' and get:
  'oocalc /home/user/big\ filename\ i\ dont\ want\ to\ type.xls'

Hope others find it as useful!

Jeremy