gnu: Honor the operating system's locale and timezone.

* gnu/system.scm (etc-directory): Add 'locale' and 'timezone'
  parameters.  Set 'LC_ALL', 'TZ', and 'TZDIR' in /etc/profile.
  (operating-system-derivation): Adjust call accordingly.
* gnu/system/vm.scm (%demo-operating-system): Change locale to
  'en_US.UTF-8'.  Add TZDATA to 'packages'.

gnu: Allow a plain list of packages in 'operating-system'.

* gnu/system.scm (union): Allow (PACKAGE OUTPUT), (PACKAGE), and
  PACKAGE.
  (<operating-system>)[packages]: Make the default a list of package?
  objects.
* gnu/system/vm.scm (%demo-operating-system): Likewise.  Add a few
  packages.

gnu: Add 'inputs' field to <user-account>; make 'shell' a monadic value.

* gnu/system/shadow.scm (<user-account>)[inputs]: New field.
  (passwd-file): Bind the 'shell' field of each account.
* gnu/system/vm.scm (%demo-operating-system): Remove 'shell' field.
* gnu/system/dmd.scm (guix-build-accounts): Store a monadic value in
  'shell'.  Add 'inputs' field.
* gnu/system.scm (operating-system-derivation): Remove 'shell' field for
  'root' account.  Add all the 'user-account-inputs' to EXTRAS.

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.