Attempt to comply with the strict namespace pollution requirements of
_POSIX_C_SOURCE. Add guards to limit visitbility of CLOCK_ and TIMER_
defines as appropriate. Only define the CLOCK_ variables relevant to the
specific standards. Move all the sharing to sys/_clock_id.h and make
time.h and sys/time.h both include that rather than copy due to the
now large number of clocks and compat defines.
Please note: The old time.h previously used these newer dates:
CLOCK_REALTIME 199506
CLOCK_MONOTONIC 200112
CLOCK_THREAD_CPUTIME_ID 200112
CLOCK_PROCESS_CPUTIME_ID 200112
but glibc defines all of these for 199309. I can't get my hands on an
actual IEEE-1003.1b standard to verify this is correct. There are
several ports that assume this value as well. So, I've taken the path of
least resistance and assumed using the glibc dates to be best.