libbe(3)/bectl(8): Make consistent with beadm

vermaden (maintainer of beadm) points out the following inconsistencies:

• "missing command" is not printed prior to usage if the error is simply a missing command; this should be obvious from the context
• "bectl rename" isn't using the "don't unmount" flag (zfs rename -u), so the active BE can't be renamed. It doesn't make sense in our context to *not* use -u, so use it.

Documentation updates reflect the above and note an inconsistency with the
'destroy' command that is consistent with other parts of the base system.

A fix for libbe(3) not properly being installed to /lib is included.
SHLIBDIR should have been added when it was moved in r337995.

Approved by: re (kib)