Support for proxy_protocol

[yrutschle]

Splitting from #482

[Proxy protocol] is a really simple proxy protocol and there are many examples of code.

Ok, there actually is a library that implements it (https://github.com/kosmas-valianos/libproxyprotocol).

I guess it could be useful to have:

• downstream support: sslh adds the proxy information when connecting to the target. We can specify this with a simple boolean in the protocols section:

protocols:
(
     { name: "ssh"; service: "ssh"; host: "localhost"; port: "22"; 
       keepalive: true; fork: true; tfo_ok: true },
     { name: "tls"; host: "localhost"; port: "4443"; proxy_protocol: true; },  # when forwarding e.g. to nginx
    [...]

that way the downstream process is aware of the client IP.

• upstream support: when sslh is placed e.g. after nginx, specified in the listen section:

listen:
(
   { host: "localhost"; port: "443"; },
   { host: "localhost"; port: "444"; proxy_protocol: true }
);

Here we'd have a "raw" connection on port 443, while 444 would expect a proxy_protocol header. This would change sslh's behaviour in two ways:

• log client IP from the proxy information instead of the socket information
• forward client IP in downstream connections that also support proxy_protocol.
  It might also be possible to use the client IP connection as transparent proxy source IP for downstream connections that wouldn't support proxy_protocol, but I'm not sure.

[yrutschle]

I understand there is no version negociation, so the protocol version needs to be specified in the configuration, so instead it'd be something like proxy_protocol: 1;.