Remove the thread argument from all mount->vfs_* function vectors,

Remove the thread argument from all mount->vfs_* function vectors,
replacing it with a ucred pointer when applicable.  This cleans up a
considerable amount of VFS function code that previously delved into
the process structure to get the cred, though some code remains.

Get rid of the compatibility thread argument for hpfs and nwfs.  Our
lockmgr calls are now mostly compatible with NetBSD (which doesn't use a
thread argument either).

Get rid of some complex junk in fdesc_statfs() that nobody uses.

Remove the thread argument from dounmount() as well as various other
filesystem specific procedures (quota calls primarily) which no longer
need it due to the lockmgr, VOP, and VFS cleanups.  These cleanups also
have the effect of making the VFS code slightly less dependant on the
calling thread's context.