[rbridge] TRILL Options

[Caitlin Bestler]

Not that the list of options described in this document needs to
be all inclusive, but I think there is one class of option at least
as probable as those cited here: a "pseudo-router" option.

This option would allow an RBridge to report congestion as though
it were a router, by encoding what a ECN capable router would have
encoded -- but without requiring the RBridge to actually modify 
L3 headers.