nar: Add 'restore-file-set', for use by build hooks.

* guix/nar.scm (&nar-invalid-hash-error, &nar-signature-error): New
  condition types.
  (&nar-error): Add 'file' and 'port' fields.
  (&nar-read-error): Remove 'port' and 'file' fields.
  (lock-store-file, unlock-store-file, finalize-store-file,
  temporary-store-directory, restore-file-set): New procedures.
* tests/nar.scm (%seed): New variable.
  (random-text): New procedure.
  ("restore-file-set (signed, valid)", "restore-file-set (missing
  signature)", "restore-file-set (corrupt)"): New tests.
* po/Makevars (XGETTEXT_OPTIONS): Add '--keyword=message'.nar fixes
* po/POTFILES.in: Add guix/nar.scm.