LIB-28: allow using numbers as devices on Mac OS X

This additionally accepts numbers as disc devices on Mac OS X. This number is taken to be the index (starting with 1) in the list of optical disc drives as given by `drutil list`.

The default for Mac is now `"1"`, with or without a disc inserted.
Direct device names are still accepted, of course.

Tracked in http://tickets.musicbrainz.org/browse/LIB-28