gnu: Add (gnu system).

* gnu/system/vm.scm (lower-inputs): Move to monads.scm.
  (qemu-image): Don't add GRUB-CONFIGURATION to the INPUTS-TO-COPY.
  (union, file-union, etc-directory): Move to gnu/system.scm.
  (%demo-operating-system): New variable.
  (system-qemu-image): Add 'os' parameter.  Rewrite in terms of
  'operating-system-derivation'.
* guix/monads.scm (lower-inputs): New procedure.
* gnu/system/grub.scm (grub-configuration-file): Change 'entries' to be
  a plain list instead of a list of monadic values.
* gnu/system.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.