Check for memset_explicit() and explicit_memset()

Check for memset_explicit() and explicit_memset()

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.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/c4701776-8d99-41da-938d-88528a3adc15%40eisentraut.org