doesn't need to be first in softc now. (It was the whole
ifnet structure itself that needed to be first in the good
old days.) Fix the respective comment accordingly.
Add xrefs to ifnet(9) in some other comments while I'm here.
Pointed out by: thompsa
imitating an Ethernet device, so vlan(4) and if_bridge(4) can be
attached to it for testing and benchmarking purposes. Its source
can be an introduction to the anatomy of a network interface driver
due to its simplicity as well as to a bunch of comments in it.
(The rest of needed changes were in my previous commit, which got
interrupted in the middle. Alas, CVS commits are not atomic.)
