MINOR: fd: centralize poll timeout computation in compute_poll_timeout()

The 4 pollers all contain the same code used to compute the poll timeout.
This is pointless, let's centralize this into fd.h. This also gets rid of
the useless SCHEDULER_RESOLUTION macro which used to work arond a very old
linux 2.2 bug causing select() to wake up slightly before the timeout.
diff --git a/src/ev_poll.c b/src/ev_poll.c
index 712bdf9..0b51e8d 100644
--- a/src/ev_poll.c
+++ b/src/ev_poll.c
@@ -193,18 +193,7 @@
 	}
 
 	/* now let's wait for events */
-	if (!exp)
-		wait_time = MAX_DELAY_MS;
-	else if (tick_is_expired(exp, now_ms)) {
-		activity[tid].poll_exp++;
-		wait_time = 0;
-	}
-	else {
-		wait_time = TICKS_TO_MS(tick_remain(now_ms, exp)) + 1;
-		if (wait_time > MAX_DELAY_MS)
-			wait_time = MAX_DELAY_MS;
-	}
-
+	wait_time = compute_poll_timeout(exp);
 	gettimeofday(&before_poll, NULL);
 	status = poll(poll_events, nbfd, wait_time);
 	tv_update_date(wait_time, status);