Add %_absolutetracknumber% variable...

...so you can number tracks sequentially on multi-track discs without
using a disc number.

See http://forums.musicbrainz.org/viewtopic.php?pid=25326 for
requirement.
Change name to albumtracknumber

to fit better with totalalbumtracks cf. tracknumber and totaltracks
D'oh - should be ~ not _
Address bitmap comment.
Switch back to %_absolutetracknumber%