lib/licenses.nix: convert to spdx.org shortnames

Done where possible, replacing links to point to spdx.org (probably stabler).
Also make libtiff use its dedicated license.

pkgs/development/libraries/libtiff/default.nix

@@ -40,9 +40,9 @@ stdenv.mkDerivation rec {
 
   doCheck = true;
 
-  meta = {
+  meta = with stdenv.lib; {
     description = "Library and utilities for working with the TIFF image file format";
     homepage = http://www.remotesensing.org/libtiff/;
-    license = "bsd";
+    license = licenses.libtiff;
   };
 }