reimplement zfsctl (.zfs) support

The current code is written on top of GFS, a library with the generic
support for writing filesystems, which was ported from illumos.
Because of significant differences between illumos VFS and FreeBSD
VFS models, both the GFS and zfsctl code were heavily modified to
work on FreeBSD. Nonetheless, they still contain quite a few ugly
hacks and bugs.

This is a reimplementation of the zfsctl code where the VFS-specific
bits are written from scratch and only the code that interacts with
the rest of ZFS is reused.

Some highlights.

We use two types of nodes, static and on-demand. The static nodes
are used for permanent directories like .zfs, .zfs/snapshot, etc. The
on-demand nodes are used for ephemeral directories that act as snapshot
mount points.
Initially only static nodes are created. Their vnodes are instantiated
when they are looked up. The on-demand nodes and vnodes are instantiated
as needed and the nodes are destroyed as soon as the corresponding
vnodes are reclaimed.
We also try very hard to ensure that uncovered snapshot vnodes do not
linger. They are supposed to become inactive as soon as they are
uncovered and we try to recycle them immediately.
When a filesystem is unmounted all snapshots under .zfs are unmounted
first, then all vnodes are flushed and finally the static .zfs nodes
are destroyed.

There are some changes outside of zfsctl code too.
z_ctldir is never used directly (as it is an opaque pointer),
zfsctl_root() has to be used instead. The function returns a locked
vnode now, so it accepts a lock flags parameter. The function can
also fail now, e.g. during force unmounting, whereas previously it
was infallible.
zfsctl_root_lookup() is retired, instead of it VOP_LOOKUP() on the .zfs
vnode (obtained with zfsctl_root) is used.

Some ideas are picked from an independent work by will.

Reviewed by: asomers, smh
MFC after: 1 month
Relnotes: maybe
Differential Revision: https://reviews.freebsd.org/D7421