Merge branch 'master' into core-updates

services: Use system groups where applicable.

* gnu/services/avahi.scm (avahi-service): Add 'system?' field to
  'user-group' form.
* gnu/services/base.scm (guix-service): Likewise.
* gnu/services/dbus.scm (dbus-service): Likewise.
* gnu/services/networking.scm (tor-service): Likewise.

services: Update to use the dmd 0.2 API.

* gnu/services/avahi.scm (avahi-service)[start]: Wrap command in a
  list.
* gnu/services/dbus.scm (dbus-service)[start]: Likewise.
* gnu/services/ssh.scm (lsh-service): Likewise.
* gnu/services/base.scm (mingetty-service)[start]: Likewise.
  (nscd-service)[start]: Likewise.
  (syslog-service)[start]: Likewise.
  (guix-service)[start]: Likewise.
  (udev-service)[start]: Use 'exec-command' instead of 'execl'.
* gnu/services/xorg.scm (slim-service)[start]: Likewise, and use
  #:environment-variables.

system: Add a 'system?' field to user accounts.

* gnu/system/shadow.scm (<user-account>)[system?]: New field.
* gnu/system.scm (user-account->gexp): Add it.
* guix/build/activation.scm (add-user): Add #:system? parameter and
  honor it.
  (activate-users+groups): Handle the 'system?' part of user tuples.
  Pass it to 'add-user'.  Don't create PROFILE-DIR when SYSTEM? is
  true.
* gnu/services/dbus.scm (dbus-service): Add 'system?' field for
  "messagebus" account.
* gnu/services/base.scm (guix-build-accounts): Likewise.
* gnu/services/avahi.scm (avahi-service): Likewise.

Merge branch 'master' into core-updates

services: Add D-Bus and Avahi.

* gnu/services/dbus.scm: New file.
* gnu/services/avahi.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add them.