Improve capture of pg_rewind stderr output

As it seems redirecting stderr to stdin (2>&1) when executing
system commands results in a SIGPIPE (141) return code, making
it impossible to determine the actual return code, redirect
stderr to a temporary file and collate the output from that.

There are possibly better ways of doing this which could
be revisited at a future date.

repmgr-action-node.c

@@ -2843,7 +2843,7 @@ do_node_rejoin(void)
 
 			if (ret == false)
 			{
-				log_error(_("unable to execute pg_rewind"));
+				log_error(_("pg_rewind execution failed"));
 				log_detail("%s", command_output.data);
 
 				termPQExpBuffer(&command_output);