Generate pg_basebackup temporary slot name using backend pid

Using the client pid can easily be non-unique when used on different
hosts. Using the backend pid should be guaranteed unique, since the
temporary slot gets removed when the client disconnects so it will be
gone even if the pid is renewed.

Reported by Ludovic Vaugeois-Pepin

src/bin/pg_basebackup/pg_basebackup.c

@@ -493,7 +493,7 @@ LogStreamerMain(logstreamer_param *param)
 	stream.replication_slot = replication_slot;
 	stream.temp_slot = param->temp_slot;
 	if (stream.temp_slot && !stream.replication_slot)
-		stream.replication_slot = psprintf("pg_basebackup_%d", (int) getpid());
+		stream.replication_slot = psprintf("pg_basebackup_%d", (int) PQbackendPID(param->bgconn));
 
 	if (format == 'p')
 		stream.walmethod = CreateWalDirectoryMethod(param->xlog, 0, do_sync);