zdoom: Mark it and its forks as nonfree.

See http://zdoom.org/wiki/License and the source code of each package.

pkgs/games/gzdoom/default.nix

···
   meta = {
     homepage = https://github.com/coelckers/gzdoom;
     description = "A Doom source port based on ZDoom. It features an OpenGL renderer and lots of new features";
+    license = stdenv.lib.licenses.unfree;
     maintainers = [ stdenv.lib.maintainers.lassulus ];
   };
 }

pkgs/games/zandronum/default.nix

···
     homepage = http://zandronum.com/;
     description = "Multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software";
     maintainers = with maintainers; [ lassulus ];
+    license = stdenv.lib.licenses.unfree;
     platforms = platforms.linux;
-    license = licenses.bsdOriginal;
   };
 }

pkgs/games/zdoom/default.nix

···
   meta = {
     homepage = http://zdoom.org/;
     description = "Enhanced port of the official DOOM source code";
+    license = stdenv.lib.licenses.unfree;
     maintainers = [ stdenv.lib.maintainers.lassulus ];
   };
 }