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

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

DEBUG_REDZONE needs DDB which needs KDB.

PR: 130324
Submitted by: Jaakko Heinonen <jh@saunalahti.fi>
MFC after: 3 days

Add buffer corruption protection (RedZone) for kernel's malloc(9).
It detects both: buffer underflows and buffer overflows bugs at runtime
(on free(9) and realloc(9)) and prints backtraces from where memory was
allocated and from where it was freed.

Tested by: kris