dhclient - Add -x and -w and adjust rc scripts

* Add the -w option which prevents dhclient from exiting on interface
  down.  dhclient will sleep 10 seconds and retry, forever.

* Add the -x option which kills any running dhclient on the specified
  interface.

* Adjust rc.d/dhclient to use the -x option to kill the dhclient instead
  of taking down the interface (which no longer kills it), and also adjust
  the default flags to pass -w.