scm_i_make_continuation takes vm and vm_cont args explicitly

scm_i_make_continuation takes vm and vm_cont args explicitly

* libguile/continuations.h:
* libguile/continuations.c (scm_i_make_continuation): Take VM and VM
  continuation arguments as well; I'm not convinced that saving all VM
  continuations was the right thing, and in any case we only ever saved
  the latest. Running a new VM should create a continuation barrier.

* libguile/stacks.c (scm_make_stack):
* libguile/vm-i-system.c (call/cc, tail-call/cc): Adapt callers.

* libguile/vm.h (scm_i_vm_capture_continuation)
  (scm_i_vm_reinstate_continuation): Change to be internal, and to only
  capture and reinstate continuations for a particular VM.