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 peers via
  IPv6
- add a hard-block for -tor to be able to remove compatibility code
- 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
- remove proxy info from RPC getinfo call
- add new getproxyinfo RPC call