pps: Round to closest integer in pps_event()

[sebastian.huber_embedded-brains.de]

Summary

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 nsec value is a time difference and not a timestamp. So, rounding
to the closest integer is probably slightly better.

[imp]

My quick algebra says this is good and computed in the right way.

sys/kern/kern_tc.c

1898

so we go from nsec = nsec / freq to nsec = (nsec + freq / 2) / freq which is nsec / freq + freq / (2 * freq) or nsec / freq + 1/2 which looks good.