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