extend core proxy options and handling

- rework the proxy handling in init to cover more cases and work more
  thoroughly
- add -proxy6 to allow setting a separate SOCKS5 proxy to reach IPv6 peers
- rework proxy data-structures to allow recognition of the base proxy
  (-proxy) to give users the ability to see, which proxy (IPv6 / Tor) is
  derived from the base proxy and which was explicitly set
- remove proxy info from RPC getinfo call
- add new getproxyinfo RPC call