pps: Round to closest integer in pps_event()

The comment above bintime2timespec() says:

  When converting between timestamps on parallel timescales of differing
  resolutions it is historical and scientific practice to round down.

However, the delta_nsec value is a time difference and not a timestamp.  Also
the rounding errors accumulate in the frequency accumulator, see hardpps().
So, rounding to the closest integer is probably slightly better.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/604

cpukit/score/src/kern_tc.c

@@ -2266,6 +2266,7 @@ pps_event(struct pps_state *pps, int event)
 #ifdef PPS_SYNC
 	if (fhard) {
 		uint64_t delta_nsec;
+		uint64_t freq;
 
 		/*
 		 * Feed the NTP PLL/FLL.
@@ -2277,7 +2278,8 @@ pps_event(struct pps_state *pps, int event)
 		tcount &= captc->tc_counter_mask;
 		delta_nsec = 1000000000;
 		delta_nsec *= tcount;
-		delta_nsec /= captc->tc_frequency;
+		freq = captc->tc_frequency;
+		delta_nsec = (delta_nsec + freq / 2) / freq;
 		hardpps(tsp, (long)delta_nsec);
 	}
 #endif