Rename (gnu services dmd) to (gnu services shepherd).

* gnu/services/dmd.scm: Rename to...
* gnu/services/shepherd.scm: ... this.
* gnu/system.scm: Use it.
* gnu/system/install.scm: Likewise.
* gnu/services/xorg.scm: Likewise.
* gnu/services/web.scm: Likewise.
* gnu/services/ssh.scm: Likewise.
* gnu/services/networking.scm: Likewise.
* gnu/services/mail.scm: Likewise.
* gnu/services/lirc.scm: Likewise.
* gnu/services/desktop.scm: Likewise.
* gnu/services/dbus.scm: Likewise.
* gnu/services/databases.scm: Likewise.
* gnu/services/base.scm: Likewise.
* gnu/services/avahi.scm: Likewise.
* guix/scripts/system.scm: Likewise.
* tests/services.scm: Likewise.
* tests/guix-system.sh: Likewise.
* doc/guix.texi (Shepherd Services): Adjust accordingly.
* gnu-system.am (GNU_SYSTEM_MODULES): Likewise.
* po/guix/POTFILES.in: Likewise.