rc.firewall: Support on-disk lists

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
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D57679