Prepare for ID3 support on userspace NFS/SMB/CIFS mounts.

Use a 128 byte buffer instead of reading 10 bytes first and then 118.

The check was added in commit 98fd9b7d, but there was never a
definition.

Obsolete class CloseSongEnumerator, which was a kludge.

Copy the query buffer to the allocated buffer, and read only the
remaining data.

Omit the file size check if we don't know the size.

Apply padding only to the fseek(), not to the chunk size.  This fixes
bogus "failed to read riff chunk" messages when the last chunk has an
odd size.

See http://bugs.musicpd.org/view.php?id=4486

This only applies to linux systems.  Here, sched_setscheduler() is
called to get realtime scheduling.  With this patch, the return value
of this function is now checked and a warning / error message is
generated if it fails.

For exception-safety.