MINOR: init: report the list of optionally available services

It's never easy to guess what services are built in. We currently have
the prometheus exporter in contrib/ which is the only extension for now.
Let's enumerate all available ones just like we do for filterr and pollers.
diff --git a/src/haproxy.c b/src/haproxy.c
index 28c72a5..6a5abcb 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -412,6 +412,8 @@
 	putchar('\n');
 	list_mux_proto(stdout);
 	putchar('\n');
+	list_services(stdout);
+	putchar('\n');
 	list_filters(stdout);
 	putchar('\n');
 }