Add one-line GPL boilerplate to numerous (but not all yet) source files.
diff --git a/archival/libunarchive/header_skip.c b/archival/libunarchive/header_skip.c
index 629fa54..53242e0 100644
--- a/archival/libunarchive/header_skip.c
+++ b/archival/libunarchive/header_skip.c
@@ -1,4 +1,7 @@
 /* vi: set sw=4 ts=4: */
+/*
+ * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
+ */
 #include <stdio.h>
 #include "unarchive.h"