mingw: add symlink-specific error codes

mingw: add symlink-specific error codes

The Win32 API calls do not set `errno`; Instead, error codes for failed
operations must be obtained via the `GetLastError()` function. Git would
not know what to do with those error values, though, which is why Git's
Windows compatibility layer translates them to `errno` values.

Let's handle a couple of symlink-related error codes that will become
relevant with the upcoming support for symlinks on Windows.

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>