daemon: Internationalize guix-daemon.

* nix/nix-daemon/guix-daemon.cc (n_, _): New macros.
  (guix_textdomain): New variable.
  (doc): Use 'n_'.
  (options): Likewise, and lowercase messages.
  (argp): Add initializer for 'argp_domain' field.
  (parse_opt): Use '_' for messages.
  (main): Likewise, and add calls to 'setlocale', 'bindtextdomain', and
  'textdomain'.
* daemon.am (guix_daemon_CPPFLAGS): Add -DLOCALEDIR.
* po/guix/Makevars (XGETTEXT_OPTIONS): Remove '--language' option.  Add
  '--keyword=n_'.
* po/guix/POTFILES.in: Add guix-daemon.cc.