Fix INIT_ON_FREE_DEFAULT_ON vs PAGE_POISONING issue #28

PAGE_POISONING is a debugging feature.
It provides less erasing than INIT_ON_FREE_DEFAULT_ON.
Join these checks with OR giving preference to INIT_ON_FREE_DEFAULT_ON.

Thanks to @madaidan for the details.

Also drop my previous recommendations about CONFIG_PAGE_POISONING_NO_SANITY
and CONFIG_PAGE_POISONING_ZERO.