Merge libbe(3)/bectl(8) from projects/bectl into head

bectl(8) is an administrative interface for working with ZFS boot
environments, intended to provide a superset of the functionality provided
by sysutils/beadm.

libbe(3) is the back-end library that the required functionality has been
pulled out into for later reuse.

These were originally written for GSoC 2017 under the mentorship of
allanjude@.

bectl(8) has proven pretty stable in my testing, with the known bug
documented in the man page.

Relnotes: yes