Rework malloc. The new default implementation is based on dlmalloc from Doug

Rework malloc.  The new default implementation is based on dlmalloc from Doug
Lea.  It is about 2x faster than the old malloc-930716, and behave itself much
better -- it will properly release memory back to the system, and it uses a
combination of brk() for small allocations and mmap() for larger allocations.
 -Erik