Don't require a writable string, and don't modify it.

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

Parse the R128_TRACK_GAIN comment string.

Call decoder_timestamp().  This is not necessary currently, but will
be as soon as we implement seeking.

This requires seeking to the end-of-stream, checking its granulepos,
and then seeking back to the previous file position.  We do this only
for local files.