ICE client: use netadr_t; stricter candidate checking

- Use netadr_t/CIPAddress almost everywhere, instead of SteamNetworkingIPAddr.
  We have two address types, the former is for internal use and the
  latter is simpler and for external API use only.
- CalcICECandidateType is now more picky.  It will reject localhost when
  not on a mock network because these addresses should never be
  involved in ICE.  Callers will now check this result and either assert
  (local candidates, we should not gather them in the first palce) or
  spew and ignore (peer canddiate or STUN/TURN server is doing something
  weird).  This means that the no-mock test should not use the mocked
  STUN/TURN server.  A STUN/TURN server running locally only makes sense
  if you are running a mocked network.

Author: zpostfacto