The firewall_type is ignored if not set in rc.conf or rc.conf.local,
after r190575 there is an option to call rc.firewall with the firewall_type
passed in as an argument.

Submitted by: David P. Discher <dpd@dpdtech.com>
MFC after: 3 weeks.
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D14286