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

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

Upgrade the netgraph vlan node to support 802.1q, encapsulation type,
PCP and CFI fields.

* Ethernet_type for VLAN encapsulation is tunable, default is 0x8100;
* PCP (Priority code point) and CFI (canonical format indicator) is
tunable per VID;
* Tunable encapsulation to support 802.1q
* Encapsulation/Decapsulation code improvements

New messages have been added for this netgraph node to support the
new features.

However, the legacy "vlan" id is still supported and compiled in by
default. It can be disabled in a future release.

TODO:

* Documentation
* Examples

PR: kern/161908
Submitted by: Ivan <rozhuk.im@gmail.com>

Use variable-sized arrays where appropriate.

Netgraph node type for IEEE 802.1Q VLAN tagging.