Implement HAProxy protocol support

This patch implements support for incoming connections wrapped in the
HAProxy protocol v1.

This is useful when running chasquid behind a HAProxy server, as it
needs the original source IP to perform SPF checks.

This patch is a reimplementation of one originally provided by Denys
Vitali in pull request #15, except the logic for the protocol handling
is moved to a new package, and the smtpsrv.Conn handling of the source
IP is simplified.

It is marked as experimental for now, since we want to give it a bit
more exposure just in case the option/api needs adjustment.

Thanks a lot to Denys Vitali (@denysvitali in github) for sending the
original patch for this, and helping test it!

internal/config/config.proto

@@ -95,4 +95,9 @@ message Config {
 	// is not, we will try to autodetect it.
 	// Example: /var/run/dovecot/auth-client
 	string dovecot_client_path = 15;
+
+	// Expect incoming SMTP connections to use the HAProxy protocol.
+	// This allows deploying chasquid behind a HAProxy server, as the
+	// address information is preserved.
+	bool haproxy_incoming = 16;
 }