DragonFlyBSD

I will commit a slightly modified version of this fix that is even more permissive. It will shortcut size + 4096 == bigbytes unconditionally, and also shortcut size >= bigbytes - (size >> 2) which should reduce geometrically inefficient reallocations as the size gets bigger and bigger.

I will also increase the _slaballoc size for reallocations > 64KB geoemetrically with 'size += size >> 3' (effectively). Not perfect, but it will greatly reduce the overhead for programs that do incremental reallocations.

-Matt