rc.firewall: Support on-disk lists

[des]

Summary

For firewall_allowservices and firewall_trusted, if an element of the
list looks like an absolute path, read the file, skipping comments and
blank lines, and treat the first word on each line as an address or
subnet to be added to the list.

We should probably be using tables instead, but this is still an
improvement over the status quo ante.

MFC after: 1 week
Relnotes: yes

[allanjude]

libexec/rc/rc.firewall

474

Should we require that files be absolute paths, so that there isn't the chance of "accidentally" loading a file that happens to have a name collision with an hostname or ip address?

[des]

check for absolute path

[des]

;;

[des]

I forgot there were two lists...