Add some basic support for measuring sleep mutex contention to the

mutex profiling code.  As with existing mutex profiling, measurement
is done with respect to mtx_lock() instances in the code, as opposed
to specific mutexes.  In particular, measure two things:

(1) Lock contention.  How often did this mtx_lock() call get made and
    have to sleep (or almost sleep) waiting for the lock.  This helps
    identify the "victims" of contention.

(2) Hold contention.  How often, while the lock was held by a thread
    as a result of this mtx_lock(), did another thread try to acquire
    the same mutex.  This helps identify the causes of contention.

I'm currently exploring adding measurement of "time waited for the
lock", but the current implementation has proven useful to me so far
so I figured I'd commit it so others could try it out.  Note that this
increases the size of mutexes when MUTEX_PROFILING is enabled, so you
might find you need to further bump UMA_BOOT_PAGES.  Fixes welcome.

The once over:	des, others

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