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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22513