Modifies the event interest for an existing subscriber.
Updates both the kernel-side `epoll` registration for `fd` and the
subscriber's own `Cell<Interest>` so that the value seen by the
handler stays in sync with what the kernel monitors.
# Errors
- [`io::ErrorKind::NotFound`](std::io::ErrorKind::NotFound) if no
subscriber is registered for `fd`.
- Otherwise, the [`io::Error`](std::io::Error) returned by
`epoll_ctl(EPOLL_CTL_MOD)`.