services: xorg: Rewrite using gexps.

* gnu/services/xorg.scm (xorg-start-command): Rewrite in terms of
  'gexp->script'.
  (xinitrc): Likewise.

services: Rewrite using gexps.

* gnu/services.scm (<service>)[inputs]: Remove.
* gnu/system.scm (links): Remove.
  (etc-directory): Add PASSWD and SHADOW to #:inputs.
  (operating-system-boot-script): Pass ETC to 'dmd-configuration-file'.
  (operating-system-derivation): Remove EXTRAS from the union.
* gnu/system/linux.scm (pam-service->configuration): Rewrite in terms of
  'gexp->derivation'.  Compute the contents on the build side.  Expect
  'arguments' to contain a list of gexps.
  (pam-services->directory): Rewrite in terms of 'gexp->derivation'.
  (unix-pam-service): Change 'arguments' to a list of one gexp.
* gnu/system/shadow.scm (<user-account>)[inputs]: Remove.
  [shell]: Change default value to a gexp.
  (passwd-file): Rewrite in terms of 'gexp->derivation'.  Compute
  contents on the build side.
* gnu/services/base.scm (host-name-service, mingetty-service,
  nscd-service, syslog-service, guix-service): Change 'start' and 'stop'
  to gexps; remove 'inputs' field.
  (guix-build-accounts): Change 'shell' field to a gexp.
* gnu/services/networking.scm (static-networking-service): Change
  'start' and 'stop' to gexps; remove 'inputs' field.
* gnu/services/xorg.scm (slim-service): Likewise.
* gnu/services/dmd.scm (dmd-configuration-file): Expect ETC to be a
  derivation.  Change 'config' to a gexp.  Use 'gexp->file' instead of
  'text-file'.
* doc/guix.texi (Defining Services): Update nscd example with gexps, and
  without 'inputs'.  Add xref to "G-Expressions".

system: Factorize (gnu system).

* gnu/system.scm (operating-system-accounts,
  operating-system-etc-directory): New procedures.
  (operating-system-derivation): Use them.
* gnu/services/base.scm (%base-services): Add 'host-name-service'
  invocation.

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.