install: Add a service to back the store with the target disk.

Fixes <http://bugs.gnu.org/18061>.
Reported by Adam Pribyl <pribyl@lowlevel.cz>.

* gnu/services/dmd.scm (dmd-configuration-file)[config]: Import (guix
  build utils).
* gnu/system/install.scm (make-cow-store, cow-store-service): New
  procedures.
  (installation-services): Use it.
  (%backing-directory): New variable.
* doc/guix.texi (System Installation): Add the 'deco start cow-store
  /mnt' phase.