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/t-18-haproxy/run.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+set -e
+. $(dirname ${0})/../util/lib.sh
+
+init
+
+mkdir -p .logs
+
+if ! haproxy -v > /dev/null; then
+	skip "haproxy binary not found"
+	exit 0
+fi
+
+# Set a 2m timeout: if there are issues with haproxy, the wait tends to hang
+# indefinitely, so an explicit timeout helps with test automation.
+timeout 2m
+
+# Launch haproxy in the background, checking config first to fail fast in that
+# case.
+haproxy -f haproxy.cfg -c
+haproxy -f haproxy.cfg > .logs/haproxy.log 2>&1 &
+
+generate_certs_for testserver
+add_user user@testserver secretpassword
+add_user someone@testserver secretpassword
+
+chasquid -v=2 --logfile=.logs/chasquid.log --config_dir=config &
+
+wait_until_ready 1025 # haproxy
+wait_until_ready 2025 # chasquid
+
+run_msmtp someone@testserver < content
+
+wait_for_file .mail/someone@testserver
+
+mail_diff content .mail/someone@testserver
+
+success