MINOR: polling: make epoll and kqueue not depend on maxfd anymore

MINOR: polling: make epoll and kqueue not depend on maxfd anymore

Maxfd is really only useful to poll() and select(), yet epoll and
kqueue reference it almost by mistake :
  - cloning of the initial FDs (maxsock should be used here)
  - max polled events, it's maxpollevents which should be used here.

Let's fix these places.