Add 'proxy' option to 'listen-on' statement

This commit extends "listen-on" statement with "proxy" options that
allows one to enable PROXYv2 support on a dedicated listener. It can
have the following values:

- "plain" to send PROXYv2 headers without encryption, even in the case
of encrypted transports.
- "encrypted" to send PROXYv2 headers encrypted right after the TLS
handshake.

lib/isccfg/namedconf.c

@@ -150,6 +150,11 @@ static cfg_type_t cfg_type_zone;
 
 static cfg_tuplefielddef_t listenon_tuple_fields[] = {
 	{ "port", &cfg_type_optional_port, 0 },
+	/*
+	 * Let's follow the protocols encapsulation order (lower->upper), at
+	 * least roughly.
+	 */
+	{ "proxy", &cfg_type_astring, CFG_CLAUSEFLAG_EXPERIMENTAL },
 	{ "tls", &cfg_type_astring, 0 },
 #if HAVE_LIBNGHTTP2
 	{ "http", &cfg_type_astring, 0 },