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.

Similar to boost::scope_exit, but fewer include dependencies.

Give MPD clients access to the error condition.

Fixes memory leak after InputStream::Check() failure.