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.go

@@ -123,6 +123,10 @@ func override(c, o *Config) {
 	if o.DovecotClientPath != "" {
 		c.DovecotClientPath = o.DovecotClientPath
 	}
+
+	if o.HaproxyIncoming {
+		c.HaproxyIncoming = true
+	}
 }
 
 // LogConfig logs the given configuration, in a human-friendly way.
@@ -141,4 +145,5 @@ func LogConfig(c *Config) {
 	log.Infof("  Mail log: %s", c.MailLogPath)
 	log.Infof("  Dovecot auth: %v (%q, %q)",
 		c.DovecotAuth, c.DovecotUserdbPath, c.DovecotClientPath)
+	log.Infof("  HAProxy incoming: %v", c.HaproxyIncoming)
 }