MEDIUM: polling: centralize polled events processing

MEDIUM: polling: centralize polled events processing

Currently, each poll loop handles the polled events the same way,
resulting in a lot of duplicated, complex code. Additionally, epoll
was the only one to handle newly created FDs immediately.

So instead, let's move that code to fd.c in a new function dedicated
to this task : fd_process_polled_events(). All pollers now use this
function.