guix-register: By default, attempt to deduplicate registered items.

* nix/guix-register/guix-register.cc (GUIX_OPT_DEDUPLICATE): New macro.
  (options): Add --no-deduplication.
  (deduplication): New variable.
  (parse_opt): Handle GUIX_OPT_DEDUPLICATE.
  (register_validity): Add 'optimize' parameter and honor it.
  (main): Move 'store' instanatiation after 'settings.nixStore'
  assignment.
* tests/guix-register.sh: Add test for deduplication.
* guix/nar.scm (finalize-store-file): Update comment above
  'register-path' call.