Minor improvements

To git-hg-diff, open_msg, track_timeout, lib/prog.sh, rfind
(wrappers/find.sh), [u]mount.media (wrappers/mount.sh), and
emerge.update (wrappers/emerge.sh).
Add getusers - lists all users on the system (file/nis/db)

As this was easist to do as a mini C program, and I don't want to be
commiting binaries, I create a wrapper called gen.sh, which bin/getusers
points to. gen.sh looks in share/src/ for getusers.* (${0##*/}.*) and
tries to compile it as share/src/gen/getusers, followed by `exec'ing it.
Add handlers for git's sparse-checkout feature (git-sparse[-clone]).