Fix NT path handling and stat in eio_windows

[avsm]

While fixing #931 and adding tests, I went into a rabbithole of fixes involving Windows symlinks. For anyone who want to follow (pun intended) in my footsteps, beware of Windows Redirect Guard which is now active on OpenSSH by default and completely destroys the symlink-using Flexlink OCaml toolchain.

This fixes the issue reported in #931 and uses Nt_path consistently (exposing some new functions there too). I found some other bugs in symlink handling too:

• The openat/unlinkat stubs checked an uninitialized handle before theNtCreateFile status, so failures were misreported.
• nofollow replaced the create options with a Win32 flag, dropping the directory/non-directory constraints so stat kinds didnt' work
• open_out ELOOP recovery ran readlink relative to the process directory rather than the sandbox root.
• openat was only partially applied inside Err.run, so Unix errors from open_in escaped unmapped.
• Opening for reading or stat could create a missing file.
• stat always passed FILE_NON_DIRECTORY_FILE, so statting a directory failed.
• stat with ~follow:false followed the link anyway; there is now a third open mode that opens the symlink, and fstat reports the target length as the size like the Posix backend does.

Best reviewed commit by commit and the testcases are separate so I could verify they failed before and worked after.

[talex5]

Thanks - this looks like a huge improvement!

[avsm]

Weird unrelated (?) error on fedora:

+++ b/_build/default/tests/.mdx/process.md.corrected
@@ -190,7 +190,7 @@ A script whose interpreter is missing reports the executable as not found:
let f = Eio.Stdenv.cwd env / "badinterp" in
Eio.Path.save f "#!/nonexistent/interpreter\n" ~create:(`Exclusive 0o700);
Fun.protect ~finally:(fun () -> Eio.Path.unlink f) (fun () -> Process.run mgr ["./badinterp"]);;
-Exception: Eio.Io Process Executable_not_found "./badinterp"
+Exception: Unix.Unix_error(Unix.EUNKNOWNERR 26, "execve", "")

[avsm]

That was a transient error, gone on re-run. This is good to go I think.

[talex5]

Unix.EUNKNOWNERR 26 is ETXTBSY. I'll investigate...