Remove ctype extension dependency (use OS check); issue #946

This uses a very basic OS check and a case-insensitive regular expression to do the same thing as the `ctype_alptha($path[0]) && $path[1] === ':' && $path[2] === '\\'`.

Paths:
```
\\NetworkComputer\Path
\\.\D:
\\.\c:
C:\Windows
C:\windows
C:/windows
c:/windows
/posix-path/
```

And so on.

I think the `i` modifier on the regular expression is safe because Windows has remained case-insensitive since its early days in DOS.