Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

When allocating memory, zero out them if we don't intend to overwrite them
all; before freeing memory, zero out them before we release it as free
heap. This will eliminate some potential information leak issue.

While there, remove the PURIFY option. There is a slight difference between
the new behavior and the old -DPURIFY behavior, with the latter initializes
memory with 0xff's. The difference between old and new approach does not
generate observable difference.

Obtained from: OpenBSD (partly).

Fix conflicts and merge into mainline