MFC r366566;r366567: Extend netstat to display TCP stack and detailed congestion state

Upstreaming the "-c" option used to show detailed per-connection
congestion control state for TCP sessions.

This is one summary patch, which adds the relevant variables into
xtcpcb. As previous "spare" space is used, these changes are ABI
compatible (an older version of netstat will simply not show
the newly available data from newer kernels, and a newer version
of netstat will only show zeroed data querying older kernels.

Reviewed by: tuexen
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26518