use explicit_bzero() to zero memory

use explicit_bzero() to zero memory

Samba uses memset_s() to zero-fill unused sensitive memory
areas.  This interface is difficult to use right, and is not
implemented in glibc due to this.  So many parts of samba
link to libreplace to get rep_memset_s().  But glibc provides
explicit_bzero(), so there's no need for this linkage.

Unfortunately there are just insane amount of similar
but non-standard functions, and upstream don't want
to stay on something simple and robust in this context.

Gbp-Pq: Name use-explicit_bzero-to-zero-memory.diff