extra references on the filesystem root vnode. This fixes bugs in
a number of filesystems that could cause forced umounts to erroneously
return EBUSY, and it centralises a lot of xxx_unmount logic.

This unfortunately requires the recompilation of any filesystem
KLDs, although I have added an extra sanity check that should help
to avoid panics if old modules are used (the filesystem may refuse
to umount, and if so, a warning message will appear on the console).

Not objected to by:	-stable

protect nullfs node hash with lockmgr().
properly release vnode if mount deadlock detected.
give nullfs node its own malloc type.

prettier (?) names, adding some const's around here, et al.

Reviewed by:	bde

calls, vnops, vfsops, both in /kern, and to individual file systems that
require a vfsop_ array entry.

Reviewed by:	eivind

* lockstatus() and VOP_ISLOCKED() gets a new process argument and a new
  return value: LK_EXCLOTHER, when the lock is held exclusively by another
  process.
* The ASSERT_VOP_(UN)LOCKED family is extended to use what this gives them
* Extend the vnode_if.src format to allow more exact specification than
  locked/unlocked.

This commit should not do any semantic changes unless you are using
DEBUG_VFS_LOCKS.

Discussed with:	grog, mch, peter, phk
Reviewed by:	peter

VFS_CHECKEXP.

Add fh(open|stat|stafs) syscalls to allow userland to query filesystems
based on (network) filehandle.

Obtained from:	NetBSD

the other XXXFS_DIAGNOSTIC options (not very) and mostly controlled
tracing of normal operation.  Use `#ifdef DEBUG' for non-diagnostics
and `#ifdef DIAGNOSTIC' for diagnostics.