Add "union tags" in mpc.el.

Add "union tags" in mpc.el.
* mpc.el: Remove backward compatibility code.
(mpc-browser-tags): Change default.
(mpc--find-memoize-union-tags): New var.
(mpc-cmd-flush, mpc-cmd-special-tag-p): New fun.
(mpc-cmd-find): Handle the case where the playlist does not exist.
Handle union-tags.
(mpc-cmd-list): Use mpc-cmd-special-tag-p.  Handle union-tags.
(mpc-cmd-add): Use mpc-cmd-flush.
(mpc-tagbrowser-tag-name): New fun.
(mpc-tagbrowser-buf): Use it.
(mpc-songs-refresh): Use cond.  Move to point-min as a fallback.