tmpfs - remove most mp->mnt_token cases, kqueue filterops are MPSAFE (2)

* Fix bug introduced in tmpfs_nrename().  The RB tree removal was not being
  guarded by the appropriate node lock.

* Also assert that the directory entry is still present.
shutdown - Add poweroff link and code

* Adds /sbin/poweroff (hardlink to shutdown), and recognize it.  This
  does basically the same thing as 'halt -p'.

Taken-from: FreeBSD
Submitted-by: robin@tsf-444-wpa-2-005.epfl.ch (Robin Hahling)
kernel - proc_token performance cleanups

* pfind()/pfindn()/zpfind() now acquire proc_token shared.

* Fix a bug in alllwp_scan().  Must hold p->p_token while scanning
  its lwp's.

* Process list scan can use a shared token, use pfind() instead of
  pfindn() and remove proc_token for individual pid lookups.

* cwd can use a shared p->p_token.

* getgroups(), seteuid(), and numerous other uid/gid access and setting
  functions need to use p->p_token, not proc_token (Repored by enjolras).
tmpfs - Fix deadlock

* Fix deadlock introduced in recent commits.  Do not recurse shared locks
  on tmpfs nodes as this can deadlock against an exclusive requester.
tmpfs - Fix readdir race

* The original cookie cache does not play nice with shared locks.
  Replace it with a second RB tree, indexed by cookie.
libhammer_stats.3: Add a missing MLINK and remove unnecessary quotes.
Add some missing MLINKS for the locale manual pages (per ctype_l.3).

While here, fix NAME and SEE ALSO sections.