[BUG] checks: don't abort when second poll returns an error

[BUG] checks: don't abort when second poll returns an error

Now that the response may be fragmented, we may receive early notifications
of aborts in return of poll(), as indicated below, which currently cause
an early error detection :

  21:11:21.036600 epoll_wait(3, {{EPOLLIN, {u32=7, u64=7}}}, 8, 993) = 1
  21:11:21.054361 gettimeofday({1268770281, 54467}, NULL) = 0
  21:11:21.054540 recv(7, "H"..., 8030, 0) = 1
  21:11:21.054694 recv(7, 0x967e759, 8029, 0) = -1 EAGAIN (Resource temporarily unavailable)
  21:11:21.054843 epoll_wait(3, {{EPOLLIN|EPOLLERR|EPOLLHUP, {u32=7, u64=7}}}, 8, 975) = 1
  21:11:21.060274 gettimeofday({1268770281, 60386}, NULL) = 0
  21:11:21.060454 close(7)                = 0

Just as in stream_sock, we must not believe poll() without attempting to receive,
which fixes the issue :

  21:11:59.402207 recv(7, "H"..., 8030, 0) = 1
  21:11:59.402362 recv(7, 0x8b5c759, 8029, 0) = -1 EAGAIN (Resource temporarily unavailable)
  21:11:59.402511 epoll_wait(3, {{EPOLLIN|EPOLLERR|EPOLLHUP, {u32=7, u64=7}}}, 8, 974) = 1
  21:11:59.407242 gettimeofday({1268770319, 407353}, NULL) = 0
  21:11:59.407425 recv(7, "TTP/1.0 200 OK\r\n"..., 8029, 0) = 16
  21:11:59.407606 recv(7, 0x8b5c769, 8013, 0) = -1 ECONNRESET (Connection reset by peer)
  21:11:59.407753 shutdown(7, 2 /* send and receive */) = -1 ENOTCONN (Transport endpoint is not connected)