dummynet: add simple gilbert-elliott channel model

Have a simple Gilbert-Elliott channel model in
dummynet to mimick correlated loss behavior of
realistic environments. This allows simpler testing
of burst-loss environments.

Reviewed By: tuexen, kp, pauamma_gundo.com, manpages
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D42980