gh-82367: Use `FindFirstFile` Win32 API in `ntpath.realpath()` (GH-110298)

* Use `FindFirstFile` Win32 API to fix a bug where `ntpath.realpath()`
breaks out of traversing a series of paths where a (handled)
`ERROR_ACCESS_DENIED` or `ERROR_SHARING_VIOLATION` occurs.
* Update docs to reflect that `ntpath.realpath()` eliminates MS-DOS
style names.