Add missing removal of network interfaces in regress cleanup targets.
Reducing the risk tests conflicting with each other.

ok bluhm@

Check if loopback interfaces used are skipped by pf as opposed of
bailing out if any loopback interface is skipped other than lo0.

Cleanup makefile so that tests are similar. Link pair to build.

Fix typo in comment.

Use setup and cleanup feature from regress framework.
Remove echo headlines. Use consistent target names.
Makefile cleanup.

Rework when tests have to be skipped due to insufficient configuration
of the regress machine.

Before running the tests, consistently check whether interfaces or
routing domains are busy.

During test setup, IPv6 duplicate address detection creates some
bogus pf states. Flush all states after the pf rules have been
set. Then the first test run will use the correct routing domains.

Create two gif(4) interfaces in different routing domains. Test
encapsulating IPv4 and IPv6 packets in IPv4 and IPv6 gif tunnels.