mallopt appropriately.
* malloc/malloc.h: Define M_PERTURB.
* malloc/malloc.c (perturb_byte): New variable.
(alloc_perturb, free_perturb): New macros.
(_int_malloc): Before returning, overwrite the memory if this is
requested.
(_int_free): Overwrite freed memory if requested.
(mALLOPt): Handle M_PERTURB.
* test-skeleton.c: Add call to mallopt with M_PERTURB command.