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