We can use either of these to implement a missing explicit_bzero().
explicit_memset() is supported on NetBSD. NetBSD hitherto didn't have a
way to implement explicit_bzero() other than the fallback variant.
memset_explicit() is the C23 standard, so we use it as first preference.
It is currently supported on:
- NetBSD 11
- FreeBSD 15
- glibc 2.43
It doesn't provide additional coverage, but as it's the new standard,
its availability will presumably grow, and eventually we'll be able to
remove some of the fallback variants.
