Parse ReplayGain info in LAME tags and use it if no ID3v2 ReplayGain tags
are found.  This is currently a bit unsafe, as apparently some LAME tags
have bogus ReplayGain values.  But I'm finding a lot of MP3s with valid
LAME tags that fail the LAME tag CRC check.  So until I figure out why
that's happening, it's an unreliable method for checking if the LAME tag is
valid.

A big thanks to tmz for writing the original patch.

git-svn-id: https://svn.musicpd.org/mpd/trunk@6798 09075e82-0dd4-0310-85a5-a0d7c8717e4f