Return unused parts of the stack to the OS

Return unused parts of the stack to the OS

* libguile/vm.h (struct scm_vm): Reorder fields.  Add "sp_max_since_gc"
  field.
* libguile/vm-engine.c (ALLOC_FRAME, RESET_FRAME):
* libguile/vm.c (vm_return_to_continuation)
  (vm_reinstate_partial_continuation, scm_call_n): In places where we
  could increase the stack height, update sp_max_since_gc.
  (vm_expand_stack): Relocate sp_max_since_gc on expansion.
  (scm_bootstrap_vm): Record the page size using gnulib's getpagesize.
  (return_unused_stack_to_os): New routine, run when marking stacks.