namecache work stage 2: move struct namecache to its own header file and

namecache work stage 2: move struct namecache to its own header file and
have vnode.h include it for now.  Re-engineer the namecache topology to make
it possible to track different parent directories and to make it possible
to namei/lookup paths using the namecache structure as the primary placeholder
rather then a directory vnode.  Add a few minor hacks to stabilize the system
that will be removed (no longer be necessary) in stage 3.  Get rid of the
leafonly sysctl and make its effect the default, but in order to avoid
doing too much in this stage it is still possible to disassociate a vnode
from its namecache entry, which a lot of filesystems (e.g. NFS) depend on
as a poor-man's way of invalidating entries.  The namecache topology itself,
however, will be left intact even if a vnode is disassociated in the middle
of a path.