string.h: Fix various symbol visibility issues.

1. Order declarations according to POSIX 2024 to make this easier to
review side-by-side with the spec.
2. Fix visibility of memccpy: XSI-only, not

string.h: Fix various symbol visibility issues.

1. Order declarations according to POSIX 2024 to make this easier to
review side-by-side with the spec.
2. Fix visibility of memccpy: XSI-only, not POSIX in general; require
_XOPEN_SOURCE, not just _POSIX_C_SOURCE.
3. Omit redundant _XOPEN_SOURCE test around stpcpy/stpncpy.
4. Hide strdup in POSIX 2001. Not POSIX (without XSI) until 2008.
5. Hide strerror_r until POSIX 2001. Can't find evidence of it in
any earlier POSIX or X/Open. (Not 100% sure on this one, maybe
someone can double-check my research.)
6. Add restrict to strlcat/strlcpy.
7. Omit redundant _XOPEN_SOURCE test around strndup and strnlen.
8. Hide strtok_r until POSIX 2001. Can't find evidence of it in
any earlier POSIX or X/Open. (Not 100% sure on this one, maybe
someone can double-check my research.)

Carry the restrict qualifiers on strlcat/strlcpy to libkern too.

Main reference:
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/string.h.html

PR standards/58804: string.h: wrong visibility for memccpy

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Add a (unused) variant of strlcat that uses strnlen and strlcpy to do the work.

handle fortify, ansify.

remove bogus _LIBC defines. broke lint.

Merge libkern + libc common files. As requested by core.