fchmodat/fstatat: fix regression with empty pathname

In 4b82229 (Cygwin: fix errno values set by readlinkat, 2023-04-18)
the code of `readlinkat()` was adjusted to align the `errno` with Linux'
behavior.

To accommodate for that, the `gen_full_path_at()` function was modified,
and the caller was adjusted to expect either `ENOENT` or `ENOTDIR` in
the case of an empty `pathname`, not just `ENOENT`.

However, `readlinkat()` is not the only caller of that helper function.

And while most other callers simply propagate the `errno` produced by
`gen_full_path_at()`, two other callers also want to special-case empty
`pathnames` much like `readlinkat()`: `fchmodat()` and `fstatat()`.

Therefore, these two callers need to be changed to expect `ENOTDIR` in
case of an empty `pathname`, too.

I noticed this issue when one of my workflows failed consistently
while trying to untar an archive containing a symbolic link and
claiming this:

    Cannot change mode to rwxr-xr-x: Not a directory

With this here fix, things start working as expected again.

Fixes: 4b82229 (Cygwin: fix errno values set by readlinkat, 2023-04-18)
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/syscalls.cc

@@ -4580,7 +4580,7 @@ fchownat (int dirfd, const char *pathname, uid_t uid, gid_t gid, int flags)
       int res = gen_full_path_at (path, dirfd, pathname);
       if (res)
 	{
-	  if (!(errno == ENOENT && (flags & AT_EMPTY_PATH)))
+	  if (!((errno == ENOENT || errno == ENOTDIR) && (flags & AT_EMPTY_PATH)))
 	    __leave;
 	  /* pathname is an empty string.  Operate on dirfd. */
 	  if (dirfd == AT_FDCWD)
@@ -4625,7 +4625,7 @@ fstatat (int dirfd, const char *__restrict pathname, struct stat *__restrict st,
       int res = gen_full_path_at (path, dirfd, pathname);
       if (res)
 	{
-	  if (!(errno == ENOENT && (flags & AT_EMPTY_PATH)))
+	  if (!((errno == ENOENT || errno == ENOTDIR) && (flags & AT_EMPTY_PATH)))
 	    __leave;
 	  /* pathname is an empty string.  Operate on dirfd. */
 	  if (dirfd == AT_FDCWD)