iv_fd: Switch ->poll() timeouts from relative to absolute.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
iv_fd: Introduce ->set_poll_timeout().

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
epoll: Implement ->set_poll_timeout() using timerfd_create(2).

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
kqueue: Implement ->set_poll_timeout() using EVFILT_TIMER.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
port: Implement ->set_poll_timeout() using timer_create(3C).

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
iv_fd: Allow poll methods to indicate whether a timeout occured.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>