LibAlias: implement RFC 4787 REQ 1 and 3 (full cone NAT)

[damjan.jov_gmail.com]

Summary

Make libalias's NAT use endpoint-independent mapping ("full cone NAT") for UDP, conforming to RFC 4787 requirements 1 and 3. All UDP packets sent out from a particular internal address:port leave via the same NAT address:port, regardless of their destination.

Also add some libalias tests, and fix other tests that broke.

Closes: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219918

Test Plan

Tests are included in the patch.

[thj]

So far this generally looks ok to me and the tests seem to work.

I think this NAT behaviour should be optional in the same way we made it a configuration option for pf.

I would like someone more familiar with libalias to weigh in, in case there is something I am missing.

sys/netinet/libalias/alias_db.c

242

Can you split this so the line is shorter

tests/sys/netinet/libalias/2_natout.c

388

Can you split this so the line is shorter

[glebius]

I really don't remember much of libalias internals. Trusting your expertise, Tom! Thanks, Damjan!