fix "--" handling when interspersed is false

[slushie]

This PR fixes an issue introduced when handling -- with app.Interspersed(false) set. The issue is caused by returning the -- as an argument token instead of skipping it. The change reorders the return code path, so that passing -- sets p.argsOnly = true and then subsequently returns p.Next(), rather than return the arg directly.

I discovered this when upgrading from v2.2.6 to v2.4.0. Previously, the -- token was always skipped by ParseContext.Next. This change reverts the behavior in 2.4.0 to match that of 2.2.6 -- after which it seems to have been accidentally changed. I've also included test cases to validate this behavior going forward.

For comparison, in v2.2.6 the relevant code is:

	// All remaining args are passed directly.
	if arg == "--" {
		p.argsOnly = true
		return p.Next()
	}

Thanks for your consideration!