New API call for path conversion

[Brian Dessent]

Corinna Vinschen wrote:

> Yes, I considered it and I don't think cygwin_internal is feasible.
> It's a sort of swiss-army knife for almost every dubious purpose a
> Cyvwin tool could have, but it's lacking a cleanliness of definition
> which I think is required for a well-defined path conversion API.

I think it's also kind of a PITA to have to pass things in va_args.  You
don't get any help from the compiler if you goof it, for example. 
There's also the code efficiency thing, where I see there's a precedent
for marking up frequently used functions with regparm.  I don't think
that's possible with varargs, but I could be wrong.

Brian