MINOR: mux: Print the list of existing mux protocols during HA startup

This is done in verbose/debug mode and when build options are reported.
diff --git a/src/haproxy.c b/src/haproxy.c
index 406d731..bf982c5 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -390,6 +390,8 @@
 
 	list_pollers(stdout);
 	putchar('\n');
+	list_mux_proto(stdout);
+	putchar('\n');
 	list_filters(stdout);
 	putchar('\n');
 }