Turn more licenses into lib.licenses style

Should eval cleanly, as far as -A tarball tells me.

Relevant: issue #2999, issue #739

pkgs/applications/networking/browsers/opera/default.nix

@@ -82,6 +82,6 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = http://www.opera.com;
     description = "Web browser";
-    license = "unfree";
+    license = stdenv.lib.licenses.unfree;
   };
 }