Sound skipping problems

[Jeff Roberson]

I have a patch that should greatly improve the sound skipping problems
people have under heavy io load.  Several people sent me traces that
showed the buf daemon running for hundreds of milliseconds with Giant
held, which can hold up the pcm code.  The patch is available at:

http://www.chesapeake.net/~jroberson/flushbuf.diff

This changes flushbufqueues to flush multiple buffers instead of one
buffer at a time.  It also places buffers we failed to flush at the end of
the list, and uses a sentinal to track our progress through the list.
This almost entirely solved the skipping problem for me.  Depending on
user feedback we can do a little more to reduce the time spent processing
here.

Cheers,
Jeff