Implement C23 memset_explicit (bug 32378)

[Paul Eggert]

On 2025-09-26 13:29, Joseph Myers wrote:
> That's a key question for review of this patch.  Should it be visible for
> __USE_MISC as well as __GLIBC_USE (ISOC23)?

The C23 function memccpy is visible if __USE_MISC, so there's reasonable 
precedent for making memset_explicit visible too (which is what I'd 
advocate anyway - I love exposure!).

I guess I'm not seeing why we'd want to be shy about visibility here.