[wmii] slock XID patch

Hi all,

Here's a silly little patch that allows one to instruct slock to print
its window's XID to stdout by passing it the -w option. I wrote it so
I could use ImageMagick to display an image in it after it starts,
e.g.:

slock -w | ID=`{read} display -window $ID $LOCK_IMG

in rc, or

slock -w | { read ID && display -window $ID $LOCK_IMG; }

in bash. Maybe this will be useful for someone else too.

- Chris