MEDIUM: fd: do not use the FD_POLL_* flags in the pollers anymore

MEDIUM: fd: do not use the FD_POLL_* flags in the pollers anymore

As mentioned in previous commit, these flags do not map well to
modern poller capabilities. Let's use the FD_EV_*_{R,W} flags instead.
This first patch only performs a 1-to-1 mapping making sure that the
previously reported flags are still reported identically while using
the closest possible semantics in the pollers.

It's worth noting that kqueue will now support improvements such as
returning distinctions between shut and errors on each direction,
though this is not exploited for now.