core: Fix the "ignore" meta command of the argparser.

* src/argparse.c (_gpgrt_argparse): Factor some code out to ...
(prepare_arg_return): new.
(_gpgrt_argparse): No missing arg error in ignore sections.

Options in an [ignore] section do not anymore lead to an error if an
argument is missing. However, if the option is also in a force
section the error is thrown.

• Signed-off-by: Werner Koch <wk@gnupg.org>