Add WITNESS support to lockmgr locking primitive.

This support tries to be as parallel as possible with other locking
primitives, but there are differences; more specifically:
- The base witness support is alredy equipped for allowing lock
  duplication acquisition as lockmgr rely on this.
- In the case of lockmgr_disown() the lock result unlocked by witness
  even if it is still held by the "kernel context"
- In the case of upgrading we can have 3 different situations:
  * Total unlocking of the shared lock and nothing else
  * Real witness upgrade if the owner is the first upgrader
  * Shared unlocking and exclusive locking if the owner is not the first
    upgrade but it is still allowed to upgrade
- LK_DRAIN is basically handled like an exclusive acquisition

Additively new options LK_NODUP and LK_NOWITNESS can now be used with
lockinit(): LK_NOWITNESS disables WITNESS for the specified lock while
LK_NODUP enable duplicated locks tracking. This will require manpages
update and a __FreeBSD_version bumping (addressed by further commits).

This patch also fixes a problem occurring if a lockmgr is held in
exclusive mode and the same owner try to acquire it in shared mode:
currently there is a spourious shared locking acquisition while what
we really want is a lock downgrade. Probabilly, this situation can be
better served with a EDEADLK failing errno return.

Side note: first testing on this patch alredy reveleated several LORs
reported, so please expect LORs cascades until resolved. NTFS also is
reported broken by WITNESS introduction. BTW, NTFS is exposing a lock
leak which needs to be fixed, and this patch can help it out if
rightly tweaked.

Tested by: kris, yar, Scot Hetzel <swhetzel at gmail dot com>

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