services: Rename 'dmd' services to 'shepherd'.

* gnu/services/shepherd.scm (dmd-root-service-type, %dmd-root-service)
  (dmd-service-type, <dmd-service>, dmd-service, dmd-service?)
  (make-dmd-service, dmd-service-documentation, dmd-service-provision)
  (dmd-service-requirement, dmd-service-respawn, dmd-service-start)
  (dmd-service-stop, dmd-service-auto-start?, dmd-service-modules)
  (dmd-service-imported-modules, dmd-service-file-name, dmd-service-file)
  (dmd-service-back-edges): Rename to...
  (shepherd-root-service-type, %shepherd-root-service, shepherd-service-type)
  (<shepherd-service>, shepherd-service, shepherd-service?)
  (make-shepherd-service, shepherd-service-documentation)
  (shepherd-service-provision, shepherd-service-requirement)
  (shepherd-service-respawn, shepherd-service-start)
  (shepherd-service-stop, shepherd-service-auto-start?)
  (shepherd-service-modules, shepherd-service-imported-modules)
  (shepherd-service-file-name, shepherd-service-file)
  (shepherd-service-back-edges): ...this
* gnu/services.scm: Adjust comments.
* gnu/services/avahi.scm (avahi-dmd-service): Rename to...
  (avahi-shepherd-service): ... this.
* gnu/services/base.scm (%root-file-system-dmd-service)
  (file-system->dmd-service-name, mapped-device->dmd-service-name)
  (dependency->dmd-service-name, file-system-dmd-service)
  (mingetty-dmd-service, nscd-dmd-service, guix-dmd-service)
  (guix-publish-dmd-service, udev-dmd-service, gpm-dmd-service): Rename to...
  (%root-file-system-shepherd-service)
  (file-system->shepherd-service-name, mapped-device->shepherd-service-name)
  (dependency->shepherd-service-name, file-system-shepherd-service)
  (mingetty-shepherd-service, nscd-shepherd-service, guix-shepherd-service)
  (guix-publish-shepherd-service, udev-shepherd-service)
  (gpm-shepherd-service): ... this.
* gnu/services/databases.scm (postgresql-dmd-service): Rename to...
  (postgresql-shepherd-service): ... this.
* gnu/services/desktop.scm (upower-dmd-service, elogind-dmd-service):
  Rename to...
  (upower-shepherd-service, elogind-shepherd-service): ... this.
* gnu/services/dbus.scm (dbus-dmd-service): Rename to...
  (dbus-shepherd-service): ... this.
* gnu/services/lirc.scm (lirc-dmd-service): Rename to...
  (lirc-shepherd-service): ... this.
* gnu/services/mail.scm (dovecot-dmd-service): Rename to...
  (dovecot-shepherd-service): ... this.
* gnu/services/networking.scm (ntp-dmd-service, tor-dmd-service)
  (bitlbee-dmd-service, wicd-dmd-service, network-manager-dmd-service): Rename to...
  (dbus-shepherd-service): ... this.
* gnu/services/ssh.scm (lsh-dmd-service): Rename to...
  (lsh-shepherd-service): ... this.
* gnu/services/web.scm (nginx-dmd-service): Rename to...
  (nginx-shepherd-service): ... this.
* gnu/services/xorg.scm (slim-dmd-service): Rename to...
  (slim-shepherd-service): ... this.
* gnu/system.scm (essential-services): Use '%shepherd-root-service'.
* gnu/system/install.scm (cow-store-service-type): Adjust accordingly.
* guix/scripts/system.scm (dmd-service-node-label, dmd-service-node-type)
  (export-dmd-graph): Likewise.
* tests/guix-system.sh: Likewise.
* tests/services.scm ("dmd-service-back-edges"): Rename to...
  ("shepherd-service-back-edges"): Adjust accordingly.
* doc/guix.texi: Likewise.
* doc/images/service-graph.dot: Use 'shepherd' service name.