rc.d/zfsbe: a new script designed for boot environment support

Currently zfsbe ensures that subordinate filesystems are mounted at the
right mount points.
The script assumes that the subordinate filesystems of a boot environment
have their canmount property set to noauto, so that they are not
automatically mounted on boot. Whereas the root filesystem is mounted
by the kernel, there was nothing to mount its subordinates.
rc.d/zfsbe fills that gap.

Discussed with: allanjude, will
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D7797