bectl(8): change jail command to execute jail(8)

The jail(8) command provides a variety of jail pseudo-parameters that are
useful to consumers of bectl, mount.devfs being the most-often-requested
paramater by bectl users.

command, exec.start, nopersist, and persist may not be specified via -o to
bectl. The command/exec.start remains passed as it always has at the end of
bectl, and persistence is dictated by -b/-U bectl jail arguments.

Submitted by: Wes Maag <jwmaag gmail com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19282