[dev] dmenu fast grab

Not sure how useful this will be, but I altered dmenu so it can grab the
X keyboard before reading input from stdin.

It works well on my netbook, where occasionally dmenu_run can take up to
a second to grab the keyboard, meaning I end up typing a load of
jibberish into the focused window, expecting it to go into dmenu.

Just an itch it scratches really.

Cheers,
Rob.