Implement C23 memset_explicit (bug 32378)

[Paul Eggert]

On 2025-09-26 12:39, Joseph Myers wrote:
> Although the function is only declared here for __GLIBC_USE (ISOC23),
> there's a reasonable argument for making it more widely available by
> inclusion in __USE_MISC.

If I'm reading the source code correctly, FreeBSD makes memset_visible 
in <string.h> by default; you must define a 
strict-conformance-to-old-standards macro like _ANSI_SOURCE, _C99_SOURCE 
or _C11_SOURCE to make it vanish.

Shouldn't we do something similar? I'm not seeing the point of being shy 
here.