Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make the "struct if_clone" opaque to users of the cloning API. Users
now use function calls:

if_clone_simple()
if_clone_advanced()

to initialize a cloner, instead of macros that initialize if_clone
structure.

Discussed with: brooks, bz, 1 year ago

Give a hint that softc can contain many things besides ifp.

We no longer embed ifnet in softc, and the pointer to ifnet
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

Introduce a new toy interface, edsc(4). It's a discard interface
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.)