CFR: patch to support creation of multiple mutex pools

The patch below enhances the mutex pool code to support the creation and
use of multiple mutex pools.  It creates one pool of sleep mutexes with
the MTX_NOWITNESS flag for use in building higher level (sx and lockmgr)
locks. It also creates another pool without MTX_NOWITNESS for general
purpose use. It can also be used to create pools of spin mutexes.

The users of the existing MTX_NOWITNESS pool are modified to use the
appropriate pool.

If this had been implemented earlier, it would have allowed witness to
catch the Giant vs. FILE_LOCK() lock order reversal that I recently
tracked down and fixed.

I've tested this only on i386, but it also passes the "make universe"
test.