Lock the temporary files after creating them to protect from a "dma -q"
run at just the wrong time causing a double delivery attempt for
the same message.

--- a/dma.c
+++ b/dma.c
@@ -225,6 +225,8 @@
 	fd = mkstemp(fn);
 	if (fd < 0)
 		return (-1);
+	if (flock(fd, LOCK_EX) == -1)
+		return (-1);
 	queue->mailfd = fd;
 	queue->tmpf = strdup(fn);
 	if (queue->tmpf == NULL) {