xflag: preserve positional arguments after -- #24

Reproducer

fs := flag.NewFlagSet("example", flag.ContinueOnError)
enabled := fs.Bool("enabled", false, "")

err := xflag.ParseToEnd(fs, []string{"--", "--enabled"})
fmt.Println(err, *enabled, fs.Args())

Current output:

<nil> true []

Expected output:

<nil> false [--enabled]

-- is the end-of-options delimiter, so everything following it should remain positional even when it looks like a flag.

flag.FlagSet.Parse strips the delimiter, after which ParseToEnd scans the remaining arguments and parses --enabled a second time. Splitting the original arguments at -- before parsing would preserve the delimiter semantics.

This affects direct users of xflag.ParseToEnd; cli.Parse already splits its arguments at -- first.