Tweaks to handle several issues:

issue15112 - refine how we test for positionals with 0 argcount
   this was raised by testing intermixed
   could add a test the emulates intermixed (set nargs=0 after the fact)
issue14191 - add nargs=SUPPRESS (but not intermixed method)
issue11354
issue13922 - only delete 1st --
    remove in -- in consume_positionals rather than get_values