use freezero() instead of memset/explicit_bzero + free. Substantially

use freezero() instead of memset/explicit_bzero + free.  Substantially
reduces conditional logic (-218, +82).

MOD_EXP_CTIME_MIN_CACHE_LINE_WIDTH cache alignment calculation bn/bn_exp.c
wasn'tt quite right.  Two other tricky bits with ASN1_STRING_FLAG_NDEF and
BN_FLG_STATIC_DATA where the condition cannot be collapsed completely.

Passes regress.  ok beck