Fix various issues in zfs life cycle, locking, and vop protocol.

- Restore some zfs locking and unlocking that got lost randomly.

- Enable use of the BSD vnode lock.  Lock order: all BSD vnode locks
are taken before all zfs internal locks.  There remains an issue with
O_EXCL, to be solved later (famous last words).  KASSERT the locking
scheme up the wazoo.

- Take our cruft out of zfs_lookup and move it to zfs_netbsd_lookup.
Restore much of the way zfs_lookup looked to make merging future
versions easier.  Disable use of the namecache for now because its
locking dance is too scary to contemplate.

- Implement BSD semantics for rename, to appease our tests.  This is
a provisional kludge; eventually we need VOP_RENAME to take a flag
specifying whether to use BSD semantics or POSIX semantics.

- Simplify zfs_netbsd_reclaim and make it work.  Now that getnewvnode
never tries to vclean anything itself, we need not worry about
recursion of ZFS_OBJ_MUTEX locks.

- Clarify and fix genfs node initialization and destruction.

zfs passes most of our atf vfs tests now, including the rename races.

Still to do:

- fix the impedance mismatch between our permissions model and zfs's;
- fix O_EXCL (nontrivial);
- throw dirconc at it and see how badly it explodes;
- find why zpool sometimes wedges itself during mkfs; and
- find why pool caches sometimes seem to get corrupted.

(riastradh)