Add error checking and support for DCB Exchange protocol (DCBX) version 1.01.

Add error checking and support for DCB Exchange protocol (DCBX) version 1.01.

From Kaladhar Musunuru <[email protected]>:

Added support for DCB Exchange protocol (DCBX) version 1.01.
http://www.ieee802.org/1/files/public/docs2008/az-wadekar-dcbx-capability-exchange-discovery-protocol-1108-v1.01.pdf

DCBX protocol exchanges control state machine and generic feature state
machine parameters as Organizationally specific TLVs.  The OUI used for
the DCBX TLV 1.01 is 0x001B21.  Following TLVs are decoded:

- Control state
- Priority Groups (PG)
- Priority-based Flow Control (PFC)
- Application Protocol (APP)

From me:

Add a bunch of additional error checking, and sort the main switch
statement by TLV code (and thus by the order in the specification).
Also update/add indications of what standards document what items.