argp-parse.c: ARGP_PARSE_ARGV0 shouldn't be ignored

[Roland McGrath]

> Well, I have yet to see where this option is useful and therefore have
> to rely on what miles wrote.

It is used in the Hurd in some places where a string of options comes from
a different place (from runtime RPCs).  It uses argz_extract on a string
and then passes the resultant vector to argp.  All these cases also use
the ARGP_NO_ERRS flag.