Fix bugs:

- read: support a tag appearing with the same value on a subset of tracks and
  nowhere else; previously this was ok if the value were differnet, but if say
  only a few tracks had VERSION=radio and the rest had no VERSION, VERSION
  would erroneously become a global tag

- write: support each track having own ALBUM with on global ALBUm (e.g. most
  tracks are 'album=foo' but a few are 'album=foo extended'); previously broke
  as ALBUm had to be global

- bonus: i noticed write was also missing ALBUMARTIST; added