Path prefix //./ in Cygwin

[Jeremy Drake]

On Thu, 12 Jun 2025, Sam Edge via Cygwin wrote:

> I would think that if you're building something against Cygwin, it's probably
> best to assume it's POSIX where only forward-slash is special and not try to
> second-guess.

This is unsafe, and actually where the rust PR started out.  If you only
treat '/' as special, a program may be tricked into allowing path
traversal with file/directory names like '..\..' or 'C:\Windows' which
are not path traversing or absolute paths in POSIX.