brcompat: Add support for bonded interfaces; avoid multiple cfg_read() call.s

brcompat: Add support for bonded interfaces; avoid multiple cfg_read() call.s

Bonded interfaces have to be looked up under the bonding.* hierarchy, but
the code here didn't know about that.  Also, this was calling cfg_read()
and cfg_write() for every interface to be deleted, which means that our
set of bridges and interfaces could change while we were iterating.  In
theory, they shouldn't have, since the file was locked, but that
assumption still made me nervous so I eliminated it.