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

ufs mounts can be used for null mounts. msdosfs and cd9660fs fixes
are trivial and will be committed shortly. Null mounts on top of nfs
filesystem will cause problems after rename() operation. This issue
also present in -current and can be fixed too.

o   Overload vop_open() in order to catch MNT_NODEV case.
o   Overload null_rename() to avoid cross layer renames. They don't work
    well atm.
o   Use macros from queue.h.

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

is an application space macro and the applications are supposed to be free
to use it as they please (but cannot).  This is consistant with the other
BSD's who made this change quite some time ago.  More commits to come.

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

Reviewed by:	eivind