Rename the identifier sz to __sz everywhere.

Fixes: a643f60c53 ("Make sure that the fortified function conditionals are constant")

It improve fortify checks for realpath, ptsname_r, wctomb, mbstowcs,
and wcstombs.  The runtime and compile checks have similar coverage as
with GCC.

Checked on aarch64, armhf, x86_64, and i686.
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>

WG14 decided to use the name C23 as the informal name of the next
revision of the C standard (notwithstanding the publication date in
2024).  Update references to C2X in glibc to use the C23 name.

This is intended to update everything *except* where it involves
renaming files (the changes involving renaming tests are intended to
be done separately).  In the case of the _ISOC2X_SOURCE feature test
macro - the only user-visible interface involved - support for that
macro is kept for backwards compatibility, while adding
_ISOC23_SOURCE.

Tested for x86_64.

commit 464d189b9622932a75302290625de84931656ec0 (origin/master, origin/HEAD)
Author: Noah Goldstein <goldstein.w.n@gmail.com>
Date:   Wed Jun 22 08:24:21 2022 -0700

    stdlib: Remove attr_write from mbstows if dst is NULL [BZ: 29265]

Incorrectly called `__mbstowcs_chk` in the NULL __dst case which is
incorrect as in the NULL __dst case we are explicitly skipping
the objsize checks.

As well, remove the `__always_inline` attribute which exists in
`__fortify_function`.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

mbstows is defined if dst is NULL and is defined to special cased if
dst is NULL so the fortify objsize check if incorrect in that case.

Tested on x86-64 linux.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

I used these shell commands:

../glibc/scripts/update-copyrights $PWD/../gnulib/build-aux/update-copyright
(cd ../glibc && git commit -am"[this commit message]")

and then ignored the output, which consisted lines saying "FOO: warning:
copyright statement not found" for each of 7061 files FOO.

I then removed trailing white space from math/tgmath.h,
support/tst-support-open-dev-null-range.c, and
sysdeps/x86_64/multiarch/strlen-vec.S, to work around the following
obscure pre-commit check failure diagnostics from Savannah.  I don't
know why I run into these diagnostics whereas others evidently do not.

remote: *** 912-#endif
remote: *** 913:
remote: *** 914-
remote: *** error: lines with trailing whitespace found
...
remote: *** error: sysdeps/unix/sysv/linux/statx_cp.c: trailing lines

The length and object size arguments were swapped around for realpath.
Also add a smoke test so that any changes in this area get caught in
future.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>