use HEAD in git URIs to point to the default branch

This makes the URIs shorter and dynamic: whatever the default branch
the repo uses will be used.

src/rc/broadcast.c

@@ -8,7 +8,7 @@
  *
  * This file is part of OpenRC. It is subject to the license terms in
  * the LICENSE file found in the top-level directory of this
- * distribution and at https://github.com/OpenRC/openrc/blob/master/LICENSE
+ * distribution and at https://github.com/OpenRC/openrc/blob/HEAD/LICENSE
  * This file may not be copied, modified, propagated, or distributed
  *    except according to the terms contained in the LICENSE file.
  */