util/memory: helper(s) to securely erase mem was reworked
Specially designated for this purpose `explicit_bzero()` function is
used in case it is available.
Otherwise well known trick with a volatile pointer to memset() is used
to prevent compiler optimization.
Relates: https://pagure.io/SSSD/sssd/issue/3956
Reviewed-by: Sumit Bose <sbose@redhat.com>