xinit: Fix logpath and dependencies for startx.

It seems that (almost?) all NixOS users start X using the services module,
because startx seems to be broken for quite some while. And it hit me while
getting to NixOS for the first time as well, so I then decided to just use the
service module.

As I'm working with multiple X servers, writing wrappers in ~/nixpkgs/config.nix
became tedious and so I decided to fix it, hopefully without breaking anything.

The fix consists of:

 * Provide a default location for the Xorg log (~/.xorg.log - hope that's okay)
 * Expose xauth through xinit to ensure purity and "unexpected behaviour", also
   known as "simply not working", because xauth isn't in the user's environment.
 * Actually provide the X binary so it doesn't have to be passed to startx every
   time.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>