Various VM stack management fixes

Various VM stack management fixes

* libguile/vm.c (vm_increase_sp): New interface, to increase the SP to
  some new level, possibly expanding the stack.
  (vm_push_sp, vm_restore_sp): Intefaces to vm_increase_sp.
  (vm_return_to_continuation): Don't throw an error if there's not
  enough space; instead, expand.
  (vm_reinstate_partial_continuation): Use the new helper.
  (return_unused_stack_to_os): Avoid off-by-one error (sp points to
  valid memory.)
  (scm_call_n): Never write beyond the sp.