Reject default token if not last

[nerab]

There can be only one default token, and it must be after all machine tokens. The code should enforce that.

[geemus]

@nerab yeah, I don't know that it makes a huge difference. ie I'm not sure we should worry too much when parsing if it isn't last. That said, we should ensure when we write netrc files that if they include default it gets written last. ie perhaps it would be best to be lenient toward input but strict on output, at least as a first pass.

[Vyazovoy]

@nerab, should this ticket be closed?