Replace recallocarray() with a realloc() + memset() combo.

Replace recallocarray() with a realloc() + memset() combo.

recallocarray(), with its guarantee that memory becoming unallocated is
explicitly discarded, is too slow. In rpki-client forming one particular
ibuf takes more then 4mins because every recallocarray() call ends up
doing a fresh malloc + memcpy + freezero call.
For sensitive data use ibuf_open() instead of ibuf_dynamic() to avoid
any memory reallocations.
OK tb@