netlink: improve RTM_GETADDR handling.

• Allow filtering by ifa_family & ifa_index.
• Add common RTM_<NEW|DEL|GET>ADDR parser
• Add tests verifying RTM_GETADDR filtering behaviour & output
• Factor out common netlink socket test methods into NetlinkTestTemplate
• Add NLMSG_DONE message handler

Reviewed By: pauamma
Differential Revision: https://reviews.freebsd.org/D37970

(cherry picked from commit c1871a3372e382bfcd46452d1d8d4f06561508cc)