support address family AF_PACKET

[mwanner]

SockAddr cannot currently represent an address of the AF_PACKET family. Therefore, support for it has been deferred and excluded from #667, which adds the getifaddrs function.

[LuoZijun]

@mwanner #813

[Susurrus]

@mwanner Please have a look at PR #813 and test it out for you. Hopefully it works for your usecase, but any feedback would be appreciated.