[music] Renamed to “m”, it's shorter.
[update] Fixed bug with updating templated configuration files.
[music] Few more actions for MPD player.

The new commands are file, path (which is synonym of the former), dir
and ls.  This commit also implements the tag action for MPD and
generalises the stuff in ~/bin/libexec.  This makes it possible to
overwrite the tag action for MPD on remote machines.