gnu: Define '%base-services'.

* gnu/services/base.scm (%base-services): New variable.
* gnu/system.scm (<operating-system>)[services]: Change the default
  value to %BASE-SERVICES.
* doc/guix.texi (Using the Configuration System): Change
  '%standard-services' to '%base-services'.

gnu: Introduce the (gnu services ...) modules.

* gnu/system/dmd.scm: Remove file.  Move contents to...
* gnu/services.scm, gnu/services/base.scm, gnu/services/dmd.scm,
  gnu/services/networking.scm, gnu/services/xorg.scm: ... here.  New
  files.
* gnu/system.scm, gnu/system/vm.scm: Adjust accordingly.
* guix/scripts/system.scm (%user-module): Likewise.
* doc/guix.texi (Using the Configuration System): Likewise.
  (Defining Services): Likewise.