Rename '/root/etc/dmd' directory and 'do-not-kill' file.

* gnu/services/base.scm (%do-not-kill-file): Rename to
  "/etc/shepherd/do-not-kill".
* gnu/build/linux-boot.scm (mount-root-file-system): Rename
  "/root/etc/dmd" to "/root/etc/shepherd".