sysutils/iocell: Allow for creating/keeping bridge interfaces

Add pull request #40 that allows for new jail parameters:

  - createbridge (default on)
  - keepbridge (default off)

createbridge controls whether the default bridge interfaces (bridge0
and bridge1) are created on jail startup if they do not exist.

keepbridge controls whether the bridge0 and bridge1 interfaces will
be destroyed on jail shutdown if they are not longer used.

Both these options prevents bridge interfaces manipulation in case
when they are being managed outside iocell.

PR:			266015
Approved by:		robak (maintainer timeout)
Differential Revision:	https://reviews.freebsd.org/D36325