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.