Checked source tree for any reference of the struct tcphdr th_x2 field,
which is part of the 12 possible TCP header flags. Changed the access and
use to these high-order bits using the accessor functions.
This should complete the compatibilty of various aspects with AccECN
(TH_AE), after the internal ipfw "re-checksum required" was moved
to use the TH_RES1 flag.
See also D37300.
No functional change.