swap the relative position of the __cplusplus closing
bracket and the doxygen end-group comment.

Signed-off-by: Chris Friedt <[email protected]>

Add a number of utility functions for manipulating struct timespec.

* timespec_add()
* timespec_compare()
* timespec_equal()
* timespec_is_valid()
* timespec_negate()
* timespec_normalize()
* timespec_sub()
* timespec_from_timeout()
* timespec_to_timeout()

If the `__builtin_add_overflow()` function is available, then the
API is mostly branchless, which should provide decent performance on
systems with an instruction cache and branch prediction. Otherwise,
manually written alternatives exist that are also perhaps more
readable.

The two functions at the end convert time durations between
representation as `struct timespec` and `k_timeout_t`.

Signed-off-by: Chris Friedt <[email protected]>

Add a timespec util testsuite. This should have reasonably high enough
coverage to be useful.

I would have preferred to add this as an architecture-independent
unit test (for the unit_testing platform) under tests/unit/timeutil but
there is an inconsistency about the size of time_t on the unit_testing
and native_sim/native platforms. On every other platform supported by
Zephyr, time_t is 64-bits. However, on those platforms, time_t is only
32-bits.

Signed-off-by: Chris Friedt <[email protected]>

Add documentation in the timeutil group for recently added functions
for validating, comparing, and manipulating `struct timespec`
objects as well as functions for converting between representation
of time durations as either `struct timespec` or `k_timeout_t`.

Signed-off-by: Chris Friedt <[email protected]>

Use the newly added timespec util functions to manipulate and
compare timespec structures with overflow detection.

Signed-off-by: Chris Friedt <[email protected]>

Add the k_clock API, comprised of:

* k_clock_gettime()
* k_clock_settime()
* k_clock_nanosleep()

along with the constants

* K_CLOCK_REALTIME
* K_CLOCK_MONOTONIC
* K_TIMER_ABSTIME

The primary motivation for this API is so that libc and other
libraries have a familiar-enough API to reach to when POSIX
is not available, since POSIX is optional in Zephyr.

By adding this API to the Zephyr kernel, we also eliminate
dependency cycles between libc and posix, as the kernel is
a mutual dependency.

Signed-off-by: Chris Friedt <[email protected]>

Remove POSIX clock_gettime() from the common libc time implementation,
since POSIX should not be a dependency for ISO C.

Signed-off-by: Chris Friedt <[email protected]>

Use the implementation of time() from the common libc, since there
it no longer pulls in POSIX.

Use is implied for minimal, newlib, and picolibc, and selected
for IAR.

Signed-off-by: Chris Friedt <[email protected]>

Reduce the dependency on POSIX by taking advantage of the newly added
k_clock_nanosleep().

Signed-off-by: Chris Friedt <[email protected]>

Use `timespec_from_timeout(K_MSEC(msec), &ts)` instead of leaning on
lazily-crafted timespecs with invalid tv_nsec fields.

Signed-off-by: Chris Friedt <[email protected]>

Presumably the time testsuite was separate from the c library set of
testsuites because it had a depedency on POSIX.

Since that dependency no longer exists, colocate the time testsuite
with the other c library testsuites.

Signed-off-by: Chris Friedt <[email protected]>

Use the newly added k_clock API in the kernel for

* clock_gettime()
* clock_settime()
* clock_nanosleep() and nanosleep()
* gettimeofday()

Signed-off-by: Chris Friedt <[email protected]>

The ISO C function time() should not depend on POSIX and this was
corrected recently via the common libc time() implementation.

Remove this dependency from the network subsystem where it has
been unduly needed for some time.

Similarly, XSI_SINGLE_PROCESS was a dependency for time() via
picolibc, because the time() implementation in picolibc relies
on the POSIX gettimeofday() call.

However, since Zephyr's common libc time() implementation no longer
depends on that, it can be removed as a dependency in the network
subsystem as well.

Signed-off-by: Chris Friedt <[email protected]>

Remove POSIX_TIMERS and XSI_SINGLE_PROCESS dependencies from the aws
cloud sample and the lwm2m client sample, as they are no longer
required.

Signed-off-by: Chris Friedt <[email protected]>