siftr: expose t_flags2 in siftr output

Replace the old snd_bwnd field which was kept for compatibility with the
t_flags2 field from the tcpcb. This exposes in siftr logs interesting
things such as ECN, PLPMTUD, Accurate ECN and if first bytes are
complete.

Reviewed by: rscheff (transport), chengc_netapp.com, debdrup (manpages)
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
X-NetApp-PR: #73
Differential Revision: https://reviews.freebsd.org/D34672