MFC r355065 - r355068: Linux epoll improvements.

r355065:
Linux epoll: Don't deregister file descriptor after EPOLLONESHOT is fired

Linux epoll does not remove descriptor after one-shot event has been triggered.
Set EV_DISPATCH kqueue flag rather then EV_ONESHOT to get the same behavior.

Required by Linux Steam client.

PR: 240590
Reported by: Alex S <iwtcex@gmail.com>
Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D22513

r355066:
Linux epoll: Check both read and write kqueue events existence in EPOLL_CTL_ADD

Linux epoll EPOLL_CTL_ADD op handler should always check registration
of both EVFILT_READ and EVFILT_WRITE kevents to deceide if supplied
file descriptor fd is already registered with epoll instance.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D22515

r355067:
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
Differential Revision: https://reviews.freebsd.org/D22516

r355068:
Linux epoll: Allow passing of any negative timeout value to epoll_wait

Linux epoll allow passing of any negative timeout value to epoll_wait()
to cause unbound blocking

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D22517