MINOR: poller: centralize poll return handling

When returning from the polling syscall, all pollers have a certain
dance to follow, made of wall clock updates, thread harmless updates,
idle time management and sleeping mask updates. Let's have a centralized
function to deal with all of this boring stuff: fd_leaving_poll(), and
make all the pollers use it.
diff --git a/src/ev_evports.c b/src/ev_evports.c
index 4d61154..301e86e 100644
--- a/src/ev_evports.c
+++ b/src/ev_evports.c
@@ -202,13 +202,7 @@
 			break;
 	} while(1);
 
-	clock_leaving_poll(wait_time, nevlist);
-
-	thread_harmless_end();
-	thread_idle_end();
-
-	if (sleeping_thread_mask & tid_bit)
-		_HA_ATOMIC_AND(&sleeping_thread_mask, ~tid_bit);
+	fd_leaving_poll(wait_time, nevlist);
 
 	if (nevlist > 0)
 		activity[tid].poll_io++;