argparse handling multiple "--" in args improperly
also 14364 - Argparse incorrectly handles '--'

This patch removes only one '--', the one that put a '-' in the
'arg_strings_pattern'.  It does this in 'consume_positionals' right
before calling 'take_action'.  As before it does not do this if
nargs is PARSER or REMAINDER.

It only removes the '--' that marked the end of options.

13922 is closed, but my patch refines it; it also fixes 14364
Maybe it should be billed as 14364 fix rather than 13922 reopen.
Merge branch 'issue13922' into composite
14364 reopens this issue)

Failure cases in TestDoubleDashRemoval no longer fail,
probably because composite handles *positionals better.