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!

test/util/lib.sh

@@ -123,6 +123,15 @@ function fexp() {
 	${UTILDIR}/fexp "$@"
 }
 
+function timeout() {
+	MYPID=$$
+	(
+		sleep $1
+		echo "timed out after $1, killing test"
+		kill -9 $MYPID
+	) &
+}
+
 function success() {
 	echo success
 }