Cross-reference libbe(3) and bectl(8).

Those two manual pages are already referencing each other in the HISTORY
sections, which people might skip. Mention those manual pages explicitly in
the SEE ALSO sections. Also, remove a reference to be(1) from libbe(3).

Reviewed by: bcr
Approved by: krion (mentor, implicit), mat (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D18136