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)
5fd4108940