Consolidate read code for timecounters and fix possible overflow in
bintime()/binuptime().

The algorithm to read the consistent snapshot of current timehand is
repeated in each accessor, including the details proper rollup
detection and synchronization with the writer. In fact there are only
two different kind of readers: one for bintime()/binuptime() which has
to do the in-place calculation, and another kind which fetches some
member from struct timehand.

Extract the logic into type-checked macros, GETTHBINTIME() for bintime
calculation, and GETTHMEMBER() for safe read of a structure' member.
This way, the synchronization is only written in bintime_off() and
getthmember().

In bintime_off(), use overflow-safe calculation of th_scale *
delta(timecounter). In tc_windup, pre-calculate the min delta value
which overflows and require slow algorithm, into the new timehands
th_large_delta member.

This part with overflow fix was written by Bruce Evans.

Reported by: Mark Millard <marklmi@yahoo.com> (the overflow issue)
Tested by: pho
Discussed with: emaste
Sponsored by: The FreeBSD Foundation (kib)
MFC after: 3 weeks