V1: add FLOW signature

This should be implementable by
  * consoles (eventually)
  * vchan connections
  * TCP connections
  * TLS

Note we don't change any of the existing signatures to include FLOW
as this would be backwards incompatable.

Signed-off-by: David Scott <dave.scott@citrix.com>
Add FLOW to V1_LWT

Signed-off-by: David Scott <dave.scott@citrix.com>
Update CHANGES

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #248 from djs55/flow2

V1: add FLOW signature