Pass argc pointer to parse_options()

This lets us keep argc up to date as the backend picks out arguments
from the argv array.
diff --git a/clients/dnd.c b/clients/dnd.c
index 4aad070..140f3f4 100644
--- a/clients/dnd.c
+++ b/clients/dnd.c
@@ -614,7 +614,7 @@
 	struct dnd *dnd;
 	int i;
 
-	d = display_create(argc, argv);
+	d = display_create(&argc, argv);
 	if (d == NULL) {
 		fprintf(stderr, "failed to create display: %m\n");
 		return -1;