guix system: Add 'init' sub-command.

* guix/scripts/system.scm (install): New procedure.
  (guix-system)[parse-option]: Remove check for extraneous arguments.
  [match-pair, option-arguments]: New procedures.
  Use 'option-arguments'.  Honor 'init'.
  (show-help): Document 'init'.
* doc/guix.texi (Invoking guix system): Document 'init'.

guix system: Add 'build' action.

* guix/scripts/system.scm (show-help): Document 'build' action.
  (guix-system): Honor 'build' action.
* doc/guix.texi (Invoking guix system): Add 'build' action.

guix system: Add 'vm-image' action and '--image-size' option.

* guix/scripts/system.scm (%options): Add --image-size.
  (%default-options): Add 'image-size'.
  (guix-system)[parse-options]: Handle the 'vm-image' action.
  Honor them.
  (show-help): Update accordingly.
* doc/guix.texi (Invoking guix system): Add 'vm-image'.

hydra: Add 'qemu-image' job.

* build-aux/hydra/demo-os.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
* build-aux/hydra/gnu-system.scm (qemu-jobs): New procedure.
  (hydra-jobs): Use it.
* guix/scripts/system.scm (read-operating-system): Export.

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

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.

Add 'guix system'.

* guix/scripts/system.scm: New file.
* Makefile.am (MODULES): Add it.
* po/POTFILES.in: Likewise.
* doc/guix.texi (Using the Configuration System): Link to "Invoking guix
  system".
  (Invoking guix system): New node.