mingw: handle symlinks to directories in `mingw_unlink()`

mingw: handle symlinks to directories in `mingw_unlink()`

The `_wunlink()` and `DeleteFileW()` functions refuse to delete symlinks
to directories on Windows; The error code would be `ERROR_ACCESS_DENIED`
in that case. Take that error code as an indicator that we need to try
`_wrmdir()` as well. In the best case, it will remove a symlink. In the
worst case, it will fail with the same error code again.

Signed-off-by: Karsten Blees <karsten.blees@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>