stdckdint-h: new module

This supports draft C23 <stdckdint.h>.
* doc/posix-headers/stdckdint.texi:
* lib/stdckdint.in.h, modules/stdckdint:
* modules/stdckdint-tests, tests/test-stdckdint.c:
New files.
* MODULES.html.sh, doc/gnulib.texi: Update for new module.
* lib/intprops-internal.h: Include <stdckdint.h> if C23 and
its macros would help and our substitute has not already
started to be included.
(_GL_INT_ADD_WRAPV, _GL_INT_SUBTRACT_WRAPV)
(_GL_INT_MULTIPLY_WRAPV): Use ckd_add, ckd_sub, ckd_mul
if they are defined and would help.
* lib/intprops-internal.h, lib/intprops.h: Improve comments.
The C23 restrictions on stdckdint macros already mostly applied to
intprops.h, so these are clarifications, not further restrictions.
* tests/test-intprops.c: If TEST_STDCKDINT is defined,
include <stdckdint.h> instead of "intprops.h", and test
it instead.
(VERIFY) [TEST_STDCKDINT]: Ignore the arg in this case.
(main) [TEST_STDCKDINT]: Skip tests irrelevant to stdckdint.h.

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