Prefer scm_call_n to scm_c_vm_run (scm_the_vm())

Prefer scm_call_n to scm_c_vm_run (scm_the_vm())

* libguile/vm.c (scm_i_capture_current_stack): Cosmetic tweak.
  (scm_call_n): Define here instead of in eval.c.  All callers of
  scm_c_vm_run were passing scm_the_vm() as the VM.  Eventually
  scm_call_n will replace scm_c_vm_run.

* libguile/eval.c: Adapt all callers.