autounmountd unload ZFS keys

Hello all,

I finally got some free time to hack on FreeBSD again.  I have a patch
that will enable autounmountd to unload ZFS encryption keys whenever it
unmounts a ZFS dataset:

https://reviews.freebsd.org/D31725

This is the first of a pair which I'm planning to do, which will enable
you to have encrypted ZFS home directories managed by autofs, which will
only have the keys loaded while a given user is logged in.  (This is a
common requirement in standards for high-security systems.)  The next
one I'm planning to work on is a pam module that will load ZFS keys upon
a successful login.