Linux epoll: Register events with zero event mask
Such an events are legal and should be interpreted as EPOLLERR | EPOLLHUP.
Register a disabled kqueue event in that case as we do not support EPOLLHUP yet.
Required by Linux Steam client.
PR: 240590
Reported by: Alex S <iwtcex@gmail.com>
Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22516