adduser(8): support creation of ZFS dataset

On systems utilizing ZFS, default to creating a ZFS dataset for a new
user's home directory if the parent directory resides on a ZFS dataset.
Add a flag that disables this behavior if the administrator explicitly
does not want it.

If run during installation from within a chroot, set mountpoint to legacy
after dataset creation and mount directly into the chroot. Then umount
and reset the mountpoint to inherit from parent.

Also support ZFS default encryption on user's home directory.

Feedback by: delphij
Reviewed by: imp, kevans
Pull Request: https://github.com/freebsd/freebsd-src/pull/881