Add placeholder functions for "repmgr $command --help"

There are now too many options to sensibly fit into general --help
output; we'll add separate output for each repmgr command, e.g.
"repmgr node --help".

repmgr-action-bdr.c

@@ -414,3 +414,12 @@ do_bdr_unregister(void)
 
 	return;
 }
+
+
+void
+do_bdr_help(void)
+{
+	print_help_header();
+
+
+}