replace callout with systimer for dummynet

[Matthew Dillon]

:Hi all,
:
:Following patch replace callout with systimer for dummynet:
:http://leaf.dragonflybsd.org/~sephe/dummynet0.diff
:
:This eliminates yet another dependency on system HZ, so when switching
:it to use mtag, I won't need to increase my test boxes HZ :)
:
:Please review it.  If no objection comes, I will commit it two days later.
:
:Best Regards,
:sephe

    This looks great!

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>