BUG/MINOR: polling: some events were not set in various pollers

BUG/MINOR: polling: some events were not set in various pollers

fdtab[].ev was only set in ev_sepoll. Unfortunately, some I/O handling
functions now rely on this, so depending on the polling mechanism, some
useless operations might have been performed, such as performing a useless
recv() when a HUP was reported.

This is a very old issue, the flags were only added to the fdtab and not
propagated into any poller. Then they were used in ev_sepoll which needed
them for the cache. It is unsure whether a backport to 1.4 is appropriate
or not.