nxcompshad/Shadow.cpp: Use EBADF instead of EBADFD (which is not portable to GNU/Hurd and GNU/kFreeBSD).

 We use the error code (and its underlying message for
 generating log errors and their textual messages. So,
 in theory, we could use any error code here. Thus,
 choosing the once closest to EBADFD while being
 portable to non-Linux systems.