core: More fixes for the new option/argument parser.

* src/argparse.c (arg_parse): Always set opt_flags.  New flag no_init.
(_gpgrt_argparse): Clear opt_flags at start.
(_gpgrt_argparser): Call arg_parse directly.  Call arg_parse with
no_init set.

The no_init flag is a minor performance feature to by pass the
initialize function which has already been called by _gpgrt_argparser.

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