extend core proxy options and handling

- rework the proxy handling in init to cover more cases and work more
  thoroughly
- extend RPC call getnetwork info to see if IPv6 and Tor proxy is the
  default proxy

See #2575 for discussion!
As said before - I'm still not convinced that `proxy-from-default` is useful information. Clients requesting from RPC just want to know the currently effective state, it shouldn't matter how it came that way. For troubleshooting command line argument interaction there's always debug.log.