kbx,w32: Disable the fd-passing.

* kbx/kbxserver.c (kbxd_start_command_handler): No fd-passing udner
Windows.

file descriptor passing does not work reliable in libassuan for
Windows and we actually don't need it here. It is not even used by
gpg or gpgsm. As soon as we enable fd-passing in gpgme for Windows
and see that it is robust enough we should back out this patch.