use tcp_[g|s]et_flags all over world

[rscheff]

Summary

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

[tuexen]

Please let this patch wait until the discussion of the accessor names and scope has settled down.