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
c8fc070608