MINOR: fd: add a new flag HAP_POLL_F_RDHUP to struct poller

MINOR: fd: add a new flag HAP_POLL_F_RDHUP to struct poller

We'll need to differenciate between pollers which can report hangup at
the same time as read (POLL_RDHUP) from the other ones, because only
these ones may benefit from the fd_done_recv() optimization. Epoll has
had support for EPOLLRDHUP since Linux 2.6.17 and has always been used
this way in haproxy, so now we only set the flag once we've observed it
once in a response. It means that some initial requests may try to
perform a second recv() call, but after the first closed connection it
will be enough to know that the second call is not needed anymore.

Later we may extend these flags to designate event-triggered pollers.