VFS messaging/interfacing work stage 7/99. BEGIN DESTABILIZATION!

VFS messaging/interfacing work stage 7/99.  BEGIN DESTABILIZATION!

Implement the infrastructure required to allow us to begin switching to the
new nlookup() VFS API.

	filedesc->fd_ncdir, fd_nrdir, fd_njdir

	    File descriptors (associated with processes) now record the
	    namecache pointer related to the current directory, root directory,
	    and jail directory, in addition to the vnode pointers.  These
	    pointers are used as the basis for the new path lookup code
	    (nlookup() and friends).

	file->f_ncp

	    File pointers may now have a referenced+unlocked namecache
	    pointer associated with them.  All fp's representing directories
	    have this attached.  This allows fchdir() to properly record
	    the ncp in fdp->fd_ncdir and friends.

	mount->mnt_ncp

	    The namecache topology for crossing a mount point works as
	    follows: when looking up a path element which is a mount point,
	    cache_nlookup() will locate the ncp for the vnode-under the
	    mount point.  mount->mnt_ncp represents the root of the mount,
	    that is the vnode-over.  nlookup() detects the mount point and
	    accesses mount->mnt_ncp to skip past the vnode-under.  When going
	    backwards (..), nlookup() detects the case and skips backwards.

	    The ncp linkages are: ncp->ncp->ncp[vnode_under]->ncp[vnode_over].
	    That is, when going forwards or backwards nlookup must explicitly
	    skip over the double-ncp when crossing a mount point.  This allows
	    us to keep the namecache topology intact across mount points.

NEW CACHE level API functions:

	cache_get()	Reference and lock a namecache entry
	cache_put()	Dereference and unlock a namecache entry
	cache_lock()	lock an already-referenced namecache entry
	cache_unlock()	unlock a lockednamecache entry

	    NOTE: namecache locks are exclusive and recursive.  These are
	    the 'namespace' locks that we will be using to guarentee namespace
	    operations such as in a CREATE, RENAME, or REMOVE.

	vfs_cache_setroot() 	Set the new system-wide root directory
	cache_allocroot()   	System bootstrap helper function to allocate
			    	 the root namecache node.

	cache_resolve()		Resolve a NCF_UNRESOLVED namecache node.  The
				namecache node should be locked on call.

	cache_setvp()		(resolver) associate a VP or create a negative
				cache entry representation for a namecache
				pointer and clear NCF_UNRESOLVED.  The
				namecache node should be locked on call.

	cache_setunresolved()	Revert a resolved namecache entry back to an
				unresolved state, disassociating any vnode
				but leaving the topology intact.  The
				namecache node should be locked on call.

	cache_vget()		Obtain the locked+refd vnode related to
				a namecache entry, resolving the entry if
				necessary.  Return ENOENT if the entry
				represents a negative cache hit.

	cache_vref()		Obtained a refd (not locked) vnode related to
				a namecache entry, as above.

	cache_nlookup()		The new namecache lookup routine.  This routine
				does a lookup and allocates a new namecache
				node (into an unresolved state) if necessary.
				Returns a namecache record whether or not
				the item can be found and whether or not it
				represents a positive or negative hit.

	cache_lookup()		OLD API CODE DEPRECATED, but must be maintained
				until everything has been converted over.
	cache_enter()		OLD API CODE DEPRECATED, but must be maintained
				until everything has been converted over.

NEW default VOPs

	vop_noresolve()		Implements a namecache resolver for VFSs
				which are still using the old VOP_LOOKUP/
				VOP_CACHEDLOOKUP API (which is all of them
				still).

	VOP_LOOKUP		OLD API CODE DEPRECATED, but must be maintained
				until everything has been converted over.
	VOP_CACHEDLOOKUP	OLD API CODE DEPRECATED, but must be maintained
				until everything has been converted over.

NEW PATHNAME LOOKUP CODE

	nlookup_init()		Similar to NDINIT, initialize a nlookupdata
				structure for nlookup() and nlookup_done().

	nlookup()		Lookup a path.  Unlike the old namei/lookup
				code the new lookup code does not do any
				fancy pre-disposition of the cache for
				create/delete, it simply looks up the requested
				path and returns the appropriate locked
				namecache pointer.  The caller can obtain the
				vnode and directory vnode, as applicable, from
				the one namecache structure that is returned.

				Access checks are done on directories leading
				up to the result but not done on the returned
				namecache node.

	nlookup_done()		Mandatory routine to cleanup a nlookupdata
				structure after it has been initialized and
				all operations have been completed on it.

	nlookup_simple()	(in progress) all-in-one wrapped new lookup.

	nlookup_mp()		helper call for resolving a mount point's
				glue NCP.  hackish, will be cleaned up later.

	nreadsymlink()		helper call to resolve a symlink.  Note that
				the namecache does not yet cache symlink data
				but the intention is to eventually do so to
				avoid having to do VFS ops to get the data.

	naccess()		Perform access checks on a namecache node
				given a mode and cred.

	naccess_va()		Perform access cheks on a vattr given a
				mode and cred.

Begin switching VFS operations from using namei to using nlookup.
In this batch:

	* mount 	(install mnt_ncp for cross-mount-point handling in
			nlookup, simplify the vfs_mount() API to no longer
			pass a nameidata structure)
	* [l]stat	(use nlookup)
	* [f]chdir	(use nlookup, use recorded f_ncp)
	* [f]chroot	(use nlookup, use recorded f_ncp)