Cleanup

Rename variables, improve comments, etc, for clarity and to better fit
my programmerly preferences.
comments
Merge branch 'master' into cleanup
Merge branch 'master' into cleanup
Merge branch 'cleanup' of github.com:rupa/z into cleanup
redirect to dev/null a bit more POSIXly
Merge commit '1016795b404ffa0d12a817e68725268f70fb756e'
Allow zsh to source z.sh more than once.
Add support for zsh <=4.3.10.
chmod -x z.sh
Null merge remote-tracking branch 'upstream/master'

This fork uses a better way to determine the current shell.