Make rump vnode interface lightweight: the only things we
really need are:

  0) provide VOP_OP in the alternate RUMP_VOP_OP namespace

  and for each op:
  1) schedule rump cpu
  2) call VOP_OP
  3) unschedule rump cpu

While here, take the opportunity to get rid of _t lossage in the
rump-exported interfaces.

(pooka)