Cygwin: path: Convert UNC path prefix back to drive letter.
- When symlink_info::check() is called with the path having drive
letter and UNC path is mounted to the drive, the path is replaced
with UNC path. With this patch, UNC path prefix is converted back
to drive letter. This fixes the issue:
https://cygwin.com/pipermail/cygwin/2021-November/250087.html
https://cygwin.com/pipermail/cygwin/2021-December/250103.html