Turn more licenses into lib.licenses style

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

Relevant: issue #2999, issue #739

pkgs/development/libraries/physfs/default.nix

@@ -17,6 +17,6 @@ stdenv.mkDerivation rec {
   meta = {
     homepage = "http://icculus.org/physfs/";
     description = "Library to provide abstract access to various archives";
-    license = "free";
+    license = stdenv.lib.licenses.free;
   };
 }