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-primary.c

@@ -489,3 +489,13 @@ do_primary_unregister(void)
 	PQfinish(primary_conn);
 	return;
 }
+
+
+
+void
+do_primary_help(void)
+{
+	print_help_header();
+
+
+}