Introduce xbps_array_add_first() to insert obj at the head of array.

The behaviour of this routine mimics the existing xbps_array_add() with
the difference that stored objects are moved to the right to insert
our object as the first element on the array.

Use this to add replaced packages in the transaction array at the head
rather than at the end, to preserve the proper sorting order.

lib/proplib_wrapper.c

@@ -112,6 +112,12 @@ xbps_array_add(xbps_array_t a, xbps_object_t obj)
 	return prop_array_add(a, obj);
 }
 
+bool
+xbps_array_add_first(xbps_array_t a, xbps_object_t obj)
+{
+	return prop_array_add_first(a, obj);
+}
+
 void
 xbps_array_remove(xbps_array_t a, unsigned int i)
 {