Move the NVIDIA support into its own module

Previously all card-specific stuff was scattered across xserver.nix
and opengl.nix, which is ugly. Now it can be kept together in a single
card-specific module. This required the addition of a few internal
options:

- services.xserver.drivers: A list of { name, driverName, modules,
  libPath } sets.

- hardware.opengl.package: The OpenGL implementation. Note that there
  can be only one OpenGL implementation at a time in a system
  configuration (i.e. no dynamic detection).

- hardware.opengl.package32: The 32-bit OpenGL implementation.

nixos/modules/module-list.nix

@@ -32,6 +32,7 @@
   ./hardware/opengl.nix
   ./hardware/pcmcia.nix
   ./hardware/video/bumblebee.nix
+  ./hardware/video/nvidia.nix
   ./installer/tools/nixos-checkout.nix
   ./installer/tools/tools.nix
   ./misc/assertions.nix