gnu: dmd: Make the 'networking' service slightly more robust.

* gnu/system/dmd.scm (static-networking-service)[start]: Sleep before
  running 'route', to leave time for the interface to come up.
  [stop]: Run "route del -net default".

gnu: vm: Add build users.

* gnu/system/shadow.scm (guix-build-accounts): New procedure.
* gnu/system/vm.scm (system-qemu-image): Use it.  Add the "guixbuild"
  group.
* gnu/system/dmd.scm (guix-service): Add 'builder-group' parameter.
  Pass 'guix-daemon' the '--build-users-group' option.

gnu: vm: Set the default networking route.

* gnu/system/dmd.scm (static-networking-service): Add #:gateway
  parameter and honor it.
* gnu/system/vm.scm (system-qemu-image): Pass #:gateway to
  'static-networking-service'.

Merge branch 'master' into core-updates

gnu: vm: Add nscd service.

* gnu/system/dmd.scm (nscd-service): New procedure.
* gnu/system/vm.scm (system-qemu-image): Use it.  Make /var/run/nscd.

gnu: vm: Add 'networking' service.

* gnu/system/vm.scm (system-qemu-image): Use
  'static-networking-service'.  Add /etc/resolv.conf.  Add Inetutils to
  the profile.
* gnu/system/dmd.scm (static-networking-service): New procedure.

gnu: vm: Add 'host-name' service.

* gnu/system/dmd.scm (host-name-service): New procedure.
  (mingetty-service): Require the 'host-name' service.
* gnu/system/vm.scm (system-qemu-image): Add the 'host-name' service.
  Set PS1 in /etc/bashrc to something sensible.

gnu: vm: Add a service for the Guix daemon.

* gnu/system/dmd.scm (guix-service): New procedure.
* gnu/system/vm.scm (system-qemu-image): Use it.

Merge branch 'master' into core-updates

gnu: QEMU images boots into dmd.

* gnu/system/dmd.scm: New file.
* gnu/system/vm.scm (system-qemu-image): Define dmd services.
  [populate]: Make var/log and etc/group.
  [boot]: Execute dmd directly.
  Add dmd and etc-group as inputs; add the inputs of dmd services.
* gnu-system.am (GNU_SYSTEM_MODULES): Add gnu/system/dmd.scm.