NETBSD: Extract part of sys/dev/clock_subr.h to sys/sys/clock.h

Preserve compatibility with code depending on clock_subr.h.
Use explicit values for sec-of-day, sec-of-month etc, to catch
quickly problems with ports that define int as 16 bit value.

Currently there are no ports with 16 bit (a candidate is PDP-11).
BS: Adapt includes of bs_access.c for NetBSD