system: Initialize $PATH before starting dmd services.

* gnu/services/dmd.scm (dmd-configuration-file): Call 'setenv' before
  starting services.

gnu: guix: Upgrade to 0.6.

* gnu/packages/package-management.scm (guix): Upgrade to 0.6.
* gnu/services/dmd.scm (dmd-configuration-file): Change /var/nix to
  /var/guix.
* gnu/system/vm.scm (operating-system-default-contents): Likewise.

services: slim: Use a system-wide xinitrc that defaults to wmaker.

* gnu/services/xorg.scm (xinitrc): New procedure.
  (slim-service)[slim.cfg]: Use it as the 'login_cmd'.

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

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.