kernel - cpu_pause() needs to be memory-modifying

* __asm __volatile isn't enough, it also needs the "memory"
  attribute to prevent gcc from optimizing out memory loads around
  loops using cpu_pause().
kernel - Implement shared namecache locks

* Currently highly experimental, so I've added a sysctl and default it
  to disabled for now.

  sysctl debug.ncp_shared_lock_disable
	0	Shared namecache locks enabled
	1	Shared namecache locks disabled (default)

* Removes most conflicts when concurrent processes are doing long path
  lookups with substantially similar prefixes.

* Also removes directory conflicts when concurrent processes are accessing
  different file names under the same directory using short paths.

* Shared mode is only used when the ncp is resolved and in a normal
  working state (this includes entries which have resolved to ENOENT).
  Otherwise the code falls back to exclusive mode.

* Shared namecache locks have three major complexities:

  (1) First, some bits of the nlookup() routine had to be rearranged to
      avoid double locking.  This is because the last namecache component
      always has to be locked exclusively, but a path such as a/b/d/.
      references the same ncp entry for both of the last two components.

  (2) Second, any lock on a namecache structure vhold()'s the related vp
      (if not NULL).  Shared locks present a particular issue where a
      second cpu may obtain a second shared lock before the first cpu
      is able to complete vhold()ing the vnode.  The vnode cannot be
      vhold()'d prior to the lock.  To deal with this an interlock was
      implemented (see NC_SHLOCK_VHOLD).

  (3) Finally, because there might be many concurrent shared lock users
      to avoid starving out an exclusive lock user we must stall further
      shared locks while an exclusive request is pending.

* The implementation specifically does not attempt to implement lock
  upgrading.  That's another can of worms that I'd rather not open.