need to work on seeking for AAC
also, don't reset cb->begin on seek to 0,
instead just set cb->end=cb->begin, works much better for
disabling seeking (like ADIF AAC's)

git-svn-id: https://svn.musicpd.org/mpd/trunk@356 09075e82-0dd4-0310-85a5-a0d7c8717e4f