REORG: clock: move the updates of cpu/mono time to clock.c

The entering_poll/leaving_poll/measure_idle functions that were hard
to classify and used to move to various locations have now been placed
into clock.c since it's precisely about time-keeping. The functions
were renamed to clock_*. The samp_time and idle_time values are now
static since there is no reason for them to be read from outside.
diff --git a/src/ev_evports.c b/src/ev_evports.c
index 23bf11f..dcc30a1 100644
--- a/src/ev_evports.c
+++ b/src/ev_evports.c
@@ -159,7 +159,7 @@
 	 * Determine how long to wait for events to materialise on the port.
 	 */
 	wait_time = wake ? 0 : compute_poll_timeout(exp);
-	sched_entering_poll();
+	clock_entering_poll();
 	activity_count_runtime();
 
 	do {
@@ -203,7 +203,7 @@
 			break;
 	} while(1);
 
-	sched_leaving_poll(wait_time, nevlist);
+	clock_leaving_poll(wait_time, nevlist);
 
 	thread_harmless_end();
 	thread_idle_end();