Hide struct socket and struct unpcb from the userland.

Violators may define _WANT_SOCKET and _WANT_UNPCB respectively and
are not guaranteed for stability of the structures. The violators
list is the the usual one: libprocstat(3) and netstat(1) internally
and lsof in ports.

In struct xunpcb remove the inclusion of kernel structure and add
a bunch of spare fields. The xsocket already has socket not included,
but add there spares as well. Embed xsockbuf into xsocket.

Sort declarations in sys/socketvar.h to separate kernel only from
userland available ones.

PR: 221820 (exp-run)