bectl(8): jail: Tear down jail by default after command exits

Add a -U flag to get back the old behavior. The new behavior is a little
more friendly to the common use cases, jail the BE and execute a script.
Having the jail torn down automatically when the script is finished, or when
you exit the shell, is a little more friendly than having to remember to
bectl ujail.

Batch mode (-b) will continue to leave the jail up, as it's assumed the
caller has other intentions.

Submitted by: Shawn Webb (partially)