ng_device: enable setting the device to non-blocking mode

Return success when FIONBIO or FIOASYNC ioctl are received in order to support
being set to non-blocking through fcntl(2). We return an error on FIOASYNC with
non-zero data argument since we do not support O_ASYNC.

Signed-off-by: Quentin Thébault <quentin.thebault@defenso.fr>
Reviewed by: imp, jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1827