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.