Add support for private VM backing.

Simply using malloc() depends on swap-backed VM, which tends to run out
on some systems and is inefficient when, as (mostly) here, it is never
free()d.  (We free NODELISTs, but nothing else.)  So, add support for a
private VM file.  If solve.vm exists, it will be mmap()ped and used as
a private memory arena, which is allocated from with a simple
never-free allocator.  On 64-bit arches - or, at least, on amd64 - this
lets us create a big (eg, 1T) file and use it to get far more VM than
the swap subsystem can normally support.

Of course, it slows down operation substantially.  But, when the
alternative is crashing....