dummynet - Poll only if operational, change default freq for vkernel

dummynet - Poll only if operational, change default freq for vkernel

* Default frequency for vkernel is now 100 hz instead of 1000 hz.
  The frequency can be changed at any time with a sysctl.

* The systimer is only configured when there are pipes or flows
  present, and will be deconfigured when there aren't.  This
  saves us from doing any unnecessary polling.

* dummynet still has some considerable holes related to SMP
  operation.  It really needs a rewrite to some degree.