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!

docs/man/chasquid.conf.5.pod

@@ -113,6 +113,15 @@ The path to dovecot's auth sockets is autodetected, but can be manually
 overridden using the C<dovecot_userdb_path> and C<dovecot_client_path> if
 needed.
 
+=item B<haproxy_incoming> (bool):
+
+B<EXPERIMENTAL>, might change in backwards-incompatible ways.
+
+If true, expect incoming SMTP connections to use the HAProxy protocol.
+This allows deploying chasquid behind a HAProxy server, as the address
+information is preserved, and SPF checks can be performed properly.
+Default: C<false>.
+
 =back
 
 =head1 SEE ALSO