More robust stack expansion and contraction

More robust stack expansion and contraction

* libguile/vm.c (allocate_stack, expand_stack): Return NULL on
  allocation failure instead of throwing an exception.  Throwing an
  exception is tricky to get right, and we need more context to do it
  correctly.
  (return_unused_stack_to_os): Try again if madvise returns -EAGAIN.  If
  madvise fails, print an error message.
  (vm_expand_stack): Abort if stack expansion fails.  We'll fix this in
  a future patch.
  (make_vm): Abort if we can't mmap a single page.