ddb: update printing of t_flags and tflags2

Update the ddb printing of t_flags and t_flags2 to the current state of
definitions in tcp_var.h.

Reviewed by: cc
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46222

(cherry picked from commit 093d9b46f4720392e53c171eaabfd7a6a8101170)