pull: Honor the standard build options.

Reported by Niall Dooley <dooleyn@gmail.com>
in <https://lists.gnu.org/archive/html/help-guix/2017-05/msg00038.html>.

* guix/scripts/pull.scm (%options): Add --dry-run and all of
%STANDARD-BUILD-OPTIONS.
(show-help): Add call to 'show-build-options-help'.
(%default-options): Add 'system', 'substitutes?', 'graft?',
'max-silent-time', and 'verbosity'.
(guix-pull)[parse-options]: Remove.
Use 'parse-command-line' instead.  Honor --dry-run.