smp: Add configure option --enable-smp.

smp: Add configure option --enable-smp.

Arrange for multiple Scheme machines, "processors" (pthreads), to
share the heap.  Each gets its own stack and "local" heap areas.
Gc-flips trace the stacks and evacuate the local heaps into the shared
heap.  The dump-band primitive now works like half a gc-flip,
evacuating the local heaps and saving the shared heap to disk before
the primitive gc daemons can start consing in a local heap again.