Currently malloc caches a number of free'ed regions up to 128k

Currently malloc caches a number of free'ed regions up to 128k
in size. This cache is indexed by size (in # of pages), so it is
very quick to check.  Some programs allocate and deallocate larger
allocations in a frantic way.  Accomodate those programs by also
keeping a cache of regions between 128k and 2M, in a cache of variable
sized regions.

Tested by many in snaps; ok deraadt@