Fix `xmalloc_readlink()` again

In e86a3ddd8 (win32: make readlink(2) implementation unconditional,
2021-02-12), we removed the special casing of `errno == ENOSYS` when
trying to follow symlinks. However, that handling really was necessary:

- When we followed a symlink, and found a non-symlink, and then called
  `readlink()` with that non-symlink, we got `errno == ENOSYS` on
  Windows (translated from `ERROR_NOT_A_REPARSE_POINT`), and we did
  want to stop the loop and return the current path in that case.
  (Noted by Johannes Schindelin.)

- When readlink() called DeviceIoControl() for files on certain
  filesystems (e.g. FAT or a CDROM) it returned `errno == ENOSYS`
  (translated from ERROR_INVALID_FUNCTION).

Revert the part of the patch which handled `ENOSYS` on Windows.

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