[NEW] sysutils/quickjail: Utility to create transient single-command jails

Quickjail is a utility to create transient single-command jails.

This utility operates much like jail(8) -c, but the jails it creates are not
persistent, unlike with jail(8). This makes quickjail more convenient for
cases where the command may need termination, which may prevent jail(8) from
cleaning up the jail.

WWW: https://git.kevans.dev/kevans/quickjail

Reviewed by: koobs (earlier version)
Differential Revision: https://reviews.freebsd.org/D24792