tcp: expose AccECN mode in TCPI

[rscheff]

Summary

For testing and validating, expose and differentiate between
classic ECN mode, and AccECN mode. Using two bits will subsequently
allow to also differentiate between pure ACE mode in AccECN (ACE=1, ECN=0),
and AccECN option mode (ACE=1,ECN=1) as that is expected to ultimately
become the default mode - thus allowing applications checking only
TCPI_OPT_ECN to deduce ecn feedback mode is active, also when AccECN+options
are enabled on a session.

Also fix an oversight and add the TFO option to TCPI.