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Linux epoll: Register events with zero event mask

Description

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