The first implementation of the buffer flushing daemon. It solves our

problem when syncer can't do its work because of vnode locks (PR1983).
This also solves our problem where bigger number of buffers results in a
much worse perfomance. In my configuration (i386, 128mb, BUFCACHEPERCENT=35)
this speedup tar -xzf ports.tar.gz in 2-4 times. In configuration with
low number of buffers and softupdates this may slowdown some operations
up to 15%.

 The major difference with current buffer cache is that new implementation
uses separate queues for dirty and clean buffers. I.e. BQ_LRU and BQ_AGE
replaced by BQ_CLEAN and BQ_DIRTY. This simplifies things a lot and
doesn't affect perfomance in a bad manner.

Thanks to art and costa for pointing on errors.

Tested by brad, millert, naddy, art, jj, camield

art, millert ok.

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