Ethernet loopback

[MikeO'Dell at BRL-VGR.ARPA]

(1) Chris is right that such functionality is useful, but
(2) There is much Ethernet hardware out there which cannot perform this
function (vendor(s) shall remain nameless for the moment).
The real issue, if there is one, is whether it is correct to assume
most hardware has listen-while-talk problems and do the loopback
in the generic ethernet code, or make it the responsibility of
individual hardware drivers.  At first glance, common sense would
dictate the latter, but there are more issues than meet the eye.
I am not claiming the current solution is correct, only that the
alternative requires careful evaluation (of code) before embracing it.

	-Mike