[SIP] Add SOCKS Reply Field

The SOCKS5 protocol specifies a Reply Field that can be very useful to tell the client what happened to that connection to the destination. From the RFC:

Reply field:
             o  X'00' succeeded
             o  X'01' general SOCKS server failure
             o  X'02' connection not allowed by ruleset
             o  X'03' Network unreachable
             o  X'04' Host unreachable
             o  X'05' Connection refused
             o  X'06' TTL expired
             o  X'07' Command not supported
             o  X'08' Address type not supported
             o  X'09' to X'FF' unassigned

With this information the server can tell the client application the proper error. This is very useful in split horizon networks (to tell whether the destination is reachable and fallback to local) or to implement selective proxying with rules (and fallback to local access)

It would be great if Shadowsocks added that.

Start with the linked RFC 1928 section 6 and review the issue discussion for the intended SOCKS5 Reply Field behavior. Identify the server and client entry points involved; done means destination failures are represented by the specified reply codes and the client can receive the resulting error.