Add mechanism to reserve the largest available block of memory below

Add mechanism to reserve the largest available block of memory below
0x04000000 at the earliest possible moment.  This maximizes the amount
of memory available for Scheme's heap.  Unfortunately, it still isn't
very much space compared to other platforms, and under Visual C++ it
isn't even as much as that provided by Watcom C/C++.  I don't know why
this is so poor, or whether there is anything to be done about it.