kernel - Refactor kern_mutex (mtx* functions)

kernel - Refactor kern_mutex (mtx* functions)

* Refactor kern_mutex in order to support asynchronous lock requests,
  which hammer2 is going to need.  kern_mutex already supports abortable
  locks.

* Add callback fields to the mtx_link structure.

* Use the mtx_link structure for shared locks in addition to exclusive locks,
  allowing asynchronous callbacks for shared locks and exclusive locks.

* Make the locking flags more deterministic.

* Redo the typedefs to be more like hammer2.  Typedef the structures rather
  than pointers so the typedef names can be used for structural embedding.