Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

- This driver will first appear in 6.3, not 7.0

MFC after: 0 days

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.