Redefine CLOCK_BOOTTIME to alias CLOCK_MONOTONIC, not CLOCK_UPTIME

The suspend-awareness situation with monotonic clocks across platforms
is kind of a mess, let's try not making it worse.

On Linux, CLOCK_MONOTONIC does NOT count suspended time, and
CLOCK_BOOTTIME was introduced to INCLUDE suspended time.

On OpenBSD, CLOCK_MONOTONIC DOES count suspended time, and CLOCK_UPTIME
was introduced to EXCLUDE suspended time.

On macOS, it's the same as OpenBSD, but with CLOCK_UPTIME_RAW.

Right now, we do not have a monotonic clock that counts suspended time.
We have CLOCK_UPTIME as a distinct ID alias, and CLOCK_BOOTTIME as a
preprocessor alias, both being effectively `CLOCK_MONOTONIC` for now.

When we introduce a suspend-aware clock in the future, it would make a
lot more sense to do it the OpenBSD/macOS way, i.e. to make
CLOCK_MONOTONIC include suspended time and make CLOCK_UPTIME exclude it,
because that's what the name CLOCK_UPTIME implies: a deviation from the
default intended for the uptime command to allow it to only show the
time the system was actually up and not suspended.

Let's change the define right now to make sure software using the define
would not end up using the ID of the wrong clock in the future, and fix
the IDs in the Linux compat code to match the expected changes too.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1824084
for more discussion.

Fixes: 155f15118a77 ("clock_gettime: Add Linux aliases for CLOCK_*")
Fixes: 25ada637362d ("Map Linux CLOCK_BOOTTIME to native CLOCK_UPTIME.")
Sponsored by: https://www.patreon.com/valpackett
Reviewed by: kib, imp
Differential Revision: https://reviews.freebsd.org/D39270

clock_id: These symbols weren't in 4.4BSD, adjust copyright

Peter Wemm added the first CLOCK_* symbols in 0f5ed9f420528 in 1997
after obtaining them from NetBSD. In NetBSD, jtc@netbsd.org committed
them in sys/sys/time.h rev 1.19 dated 1996/11/15, along with all the
system calls associated with 1003.1b. FreeBSD's values are, however,
different than NetBSD's today. The USL/UCB lawsuit was settled in 1994,
so these couldn't have been derived from material provided to University
of California covered in that settlement. This file does not need the
settlement disclaimer.

Furthermore, I rewrote most of the code (except the symbols and their
values) when merging it from time.h and sys/time.h. Most of the creative
content of the file is new, so update copyright to reflect that.

Reviewed by: kaktus
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31369

clock_gettime: Add Linux aliases for CLOCK_*

Linux standardized what we call CLOCK_{REALTIME,MONOTONIC}_FAST as
CLOCK_{REALTIME,MONOTONIC}_COARSE. In addition, Linux spells
CLOCK_UPTIME as CLOCK_BOOTTIME.

Add aliases to time.h and document these new aliases in
clock_gettime(2).

Reviewed by: vangyzen, kib (prior), dchagin (prior)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30988

time.h: reduce CLOCK_ namespace pollution, move to _clock_id.h

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. glibc uses this date for all
these values, however, only CLOCK_REALTIME was in IEEE 1003.1b. Add a
comment about this to document it. A large number of programs and
libraries assume that these will be defined for _POSIX_C_SOURCE =
199309.

In addition, leak CLOCK_UPTIME_FAST for the pocl package until it can be
updated to use a simple CLOCK_MONOTONIC.

Reviewed by: kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31056