Add "repmgr standby follow --upstream-node-id"

In an automatic failover situation, after a standby has been promoted
there's a risk the original primary may become available again before
"standby follow" is issued on another standby node, in which case "standby
follow" will reconnect to the original primary.

As the standby's repmgrd will have received a notification from the new
primary, it will know the primary's ID and can therefore explicitly
direct "standby follow" to follow that primary.

repmgr-client.c

@@ -1355,6 +1355,7 @@ check_cli_parameters(const int action)
 		{
 			case STANDBY_CLONE:
 			case STANDBY_REGISTER:
+			case STANDBY_FOLLOW:
 				break;
 			default:
 				item_list_append_format(&cli_warnings,