major code change for .zfs

Please review and test a change to .zfs code that is intended to make the code
aligned with FreeBSD VFS and, as such, more stable:
https://reviews.freebsd.org/D7421

The change removes two features.
.zfs/shares is gone because it was unused on FreeBSD anyway.  We can restore
that when we need it.
An ability to take a snapshot by creating a directory under .zfs/snapshot is
removed.  I hope that you didn't use it.  Please do not start using it now :-)
Again, this feature can be restored with some work.
The reason I removed it is that its companion features of destroying and
renaming snapshots were already missing on FreeBSD, and properly implementing
the feature required some more work.

-- 
Andriy Gapon