VFS messaging/interfacing work stage 8/99: Major reworking of the vnode

VFS messaging/interfacing work stage 8/99: Major reworking of the vnode
interlock and other miscellanious things.  This patch also fixes FS
corruption due to prior vfs work in head.  In particular, prior to this
patch the namecache locking could introduce blocking conditions that
confuse the old vnode deactivation and reclamation code paths.  With
this patch there appear to be no serious problems even after two days
of continuous testing.

* VX lock all VOP_CLOSE operations.
* Fix two NFS issues.  There was an incorrect assertion (found by
  David Rhodus), and the nfs_rename() code was not properly
  purging the target file from the cache, resulting in Stale file
  handle errors during, e.g. a buildworld with an NFS-mounted /usr/obj.
* Fix a TTY session issue.  Programs which open("/dev/tty" ,...) and
  then run the TIOCNOTTY ioctl were causing the system to lose track
  of the open count, preventing the tty from properly detaching.
  This is actually a very old BSD bug, but it came out of the woodwork
  in DragonFly because I am now attempting to track device opens
  explicitly.
* Gets rid of the vnode interlock.  The lockmgr interlock remains.
* Introduced VX locks, which are mandatory vp->v_lock based locks.
* Rewrites the locking semantics for deactivation and reclamation.
  (A ref'd VX lock'd vnode is now required for vgone(), VOP_INACTIVE,
  and VOP_RECLAIM).  New guarentees emplaced with regard to vnode
  ripouts.
* Recodes the mountlist scanning routines to close timing races.
* Recodes getnewvnode to close timing races (it now returns a
  VX locked and refd vnode rather then a refd but unlocked vnode).
* Recodes VOP_REVOKE- a locked vnode is now mandatory.
* Recodes all VFS inode hash routines to close timing holes.
* Removes cache_leaf_test() - vnodes representing intermediate
  directories are now held so the leaf test should no longer be
  necessary.
* Splits the over-large vfs_subr.c into three additional source
  files, broken down by major function (locking, mount related,
  filesystem syncer).

* Changes splvm() protection to a critical-section in a number of
  places (bleedover from another patch set which is also about to be
  committed).

Known issues not yet resolved:

* Possible vnode/namecache deadlocks.
* While most filesystems now use vp->v_lock, I haven't done a final
  pass to make vp->v_lock mandatory and to clean up the few remaining
  inode based locks (nwfs I think and other obscure filesystems).
* NullFS gets confused when you hit a mount point in the underlying
  filesystem.
* Only UFS and NFS have been well tested
* NFS is not properly timing out namecache entries, causing changes made
  on the server to not be properly detected on the client if the client
  already has a negative-cache hit for the filename in question.

Testing-by: David Rhodus <sdrhodus@gmail.com>,
	    Peter Kadau <peter.kadau@tuebingen.mpg.de>,
	    walt <wa1ter@myrealbox.com>,
	    others