Extend netstat to display TCP stack and detailed congestion state

Adding 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.

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