libalias: Add support for EIM NAT

Add support for endpoint-independent mapping ("full cone NAT") in
Libalias's UDP NAT.

This conforms 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.

Add some libalias tests and supporting defines.

Reviewed by: igoro, thj
Differential Revision: https://reviews.freebsd.org/D46689D