extend core proxy options and handling

- rework the proxy handling in init to cover more cases and work more
  thoroughly
- rework proxy data-structures to allow recognition of the default proxy
  (-proxy) to give users the ability to see, which proxy (IPv6 / Tor) is
  derived from the default proxy and which was explicitly set
- cleanup RPC call getnetwork info
- remove unneeded (int) casts before CLIENT_VERSION and PROTOCOL_VERSION
  getnetworkinfo RPC calls

See #2575 for discussion!