Hide struct inpcb, struct tcpcb from the userland.

This is a painful change, but it is needed. On the one hand, we avoid
modifying them, and this slows down some ideas, on the other hand we still
eventually modify them and tools like netstat(1) never work on next version of
FreeBSD. We maintain a ton of spares in them, and we already got some ifdef
hell at the end of tcpcb.

Details:

• Hide struct inpcb, struct tcpcb under _KERNEL || _WANT_FOO.
• Make struct xinpcb, struct xtcpcb pure API structures, not including kernel structures inpcb and tcpcb inside. Export into these structures the fields from inpcb and tcpcb that are known to be used, and put there a ton of spare space.
• Make kernel and userland utilities compilable after these changes.
• Bump __FreeBSD_version.

Reviewed by: rrs, gnn
Differential Revision: D10018