"standby follow": verify status of follow target

This commit adds infrastruture for repmgr to be able to check
whether one standby can attach to another node, regardless whether
it is a standby or a primary.

This is intended to prevent a node from attempting to follow a
node whose timeline has diverged. The --dry-run option makes
it possible to test a follow operation before it is carried out.

As a useful side-effect this makes it possible for a standby to
follow another standby.

This is an initial implementation; documentation and possibly
further changes to follow.

repmgr-action-node.c

@@ -2301,7 +2301,14 @@ do_node_rejoin(void)
 
 	initPQExpBuffer(&follow_output);
 
+	/*
+	 * do_standby_follow_internal() can handle situations where the follow
+	 * target is not the primary, so requires database handles to both
+	 * (even if they point to the same node). For the time being,
+	 * "node rejoin" will only attatch a standby to the primary.
+	 */
 	success = do_standby_follow_internal(upstream_conn,
+										 upstream_conn,
 										 &primary_node_record,
 										 &follow_output,
 										 &follow_error_code);