api inconsistency Epoll::modify and Epoll::add

[David Hotham]

Epoll::add takes mut event: EpollEvent, whereas Epoll::modify takes event: &mut EpollEvent

seems like an odd difference, am I missing a reason that this is sensible?

[David Hotham]

#1882, cc @JonathanWoollett-Light

[Jonathan Woollett-Light]

There is no strong reason.

Epoll::add could takes &mut EpollEvent,it may make sense to make a PR for this.