Although most of the NIC drivers are epoch ready, due to peer pressure
switch over to opt-in instead of opt-out for epoch.

Instead of IFF_NEEDSEPOCH, provide IFF_KNOWSEPOCH. If driver marks
itself with IFF_KNOWSEPOCH, then ether_input() would not enter epoch
when processing its packets.

Now this will create recursive entrance in epoch in >90% network
drivers, but will guarantee safeness of the transition.

Mark several tested drivers as IFF_KNOWSEPOCH.

Reviewed by: hselasky, jeff, bz, gallatin
Differential Revision: https://reviews.freebsd.org/D23674