Don't allow abbreviated long options in getopt

I just noticed that our getopt implementation, taken from glibc and modified for wchar, allows unambiguous long option abbreviations.

That means `string match --reg` works even tho the option is `--regex`.

I don't think I have ever seen anyone use this on purpose.

Personally, I consider this to be a terrible design decision because it means that adding new long flags can break previous calls (e.g if we added `--regex-dialect` it would break all uses of `string match --reg`), and would like to remove it.

This is likely to be controversial, and the change would have to be backwards-incompatible (although, like I said, I have never seen anyone use this on purpose).