[For discussion] Extract a FLOW signature from TCPv4

Ideally as many things as possible should implement this. I have in
mind:
  * CONSOLEs
  * VCHANs
  * TCP connections
  * TLS connections

Unfortunately the lower levels of the network stack don't quite fit this
pattern because they typically have:

  * write (good)
  * writev (good)
  * listen ~one_cb ~two_cb ... ~n_cb (bad)

Signed-off-by: David Scott <dave.scott@citrix.com>

NB this is a backwards compatible change (unless adding 'FLOW' causes a namespace problem somewhere)