No longer access th_flags (and th_x2) directly, but use
the accessor functions throughout world, also to give
and example for handling high-order flags.

In the process, add support for testing ECN flags too.

ipmon: log TH_AE flag as "a" - TH_ACK remains "A"
libipf: print "a" for TH_AE
iplang: add support for TCP ECN header flags
ipsend: add TCP ECN support
ppp: use tcphdr.flags accessor