libbe(3): Change be_mount to mount/unmount child datasets

This set of changes is geared towards making bectl respect deep boot
environments when they exist and are mounted. The deep BE composition
functionality (bectl add) remains disabled for the time being. This set of
changes has no effect for the average user. but allows deep BE users to
upgrade properly with their current setup.

libbe(3): Open the target boot environment and get a zfs handle, then pass
that with the target mountpoint to be_mount_iter; If the BE_MNT_DEEP flag is
set call zfs_iter_filesystems and mount the child datasets.

Similar logic is employed when unmounting the datasets, save for children
are unmounted first.

bectl(8): Change bectl_cmd_jail to pass the BE_MNT_DEEP flag when
calling be_mount as well as call be_unmount when cleaning up after the
jail has exited instead of umount(2) directly.

PR: 234795
Submitted by: Wes Maag <jwmaag_gmail.com> (test additions by kevans)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18796