When libvorbis knows that a song is seekable, it seeks around like
crazy in the file before starting to decode it.  This is very
expensive on remote HTTP resources, and delays MPD for 10 or 20
seconds.

This patch disables seeking on remote songs, because the advantages of
quickly playing a song seem to weigh more than the theoretical ability
of seeking for most MPD users.  If users feel this feature is needed,
we will make a configuration option for that.