The th_bintime, th_microtime and th_nanotime members of the timehand
all cache the last system time (uptime + boottime). Only the format
differs. Do not re-calculate the bintime and simply use the value
used to calculate the microtime and nanotime.

Group all the updates under the relevant comment. Remove obsoleted
XXX part.

Submitted by: Sebastian Huber <sebastian.huber@embedded-brains.de>
MFC after: 1 week