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

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

Add a netgraph node to handle ATM LLC encapsulation. This currently handles
ethernet (tested) and FDDI (not tested). The main use for this is on ADSL (or
other ATM) connections where bridged ethernet is used, PPPoE being a prime
example.

There is no manual page as yet, I will write one shortly.

Reviewed by: harti