rc.firewall: default loopback rules are set up even for custom file

[Andriy Gapon]

I've just realized that I see in releng/7 something that I did not see
in releng/6 - even if I use a file with custom rules in firewall_type I
still get default loopback rules installed.
I think that this is not correct, I am using custom rules exactly
because I want to control *everything* (e.g. all deny rules come with
log logamount xxx).

-- 
Andriy Gapon