Further minor cleanups to UNIX domain sockets:

- Staticize and locally prototype functions uipc_ctloutput(),
  unp_dispose(), unp_init(), and unp_externalize(), none of which have
  been required outside of uipc_usrreq.c since uipc_proto.c was removed.
- Remove stale prototype for uipc_usrreq(), which has not existed in the
  code since 1997
- Forward declare and staticize uipc_usrreqs structure in uipc_usrreq.c
  and not un.h.
- Comment on why uipc_connect2() is still non-static -- it is used
  directly by fifofs.
- Remove stale comments, tidy up whitespace.

Use soconnect2() rather than directly invoking uipc_connect2() to
interconnect two UNIX domain sockets.

Now that portalfs doesn't directly invoke uipc_connect2(), make it a
static symbol.

Remove stale comment (and XXX saying so) about why we zero the file
descriptor pointer in unp_freerights: we can no longer recurse into
unp_gc due to unp_gc being invoked in a deferred way, but it's still
a good idea.

Remove stale comment: while uipc_connect2() was, until recently, not
static so it could be used by fifofs (actually portalfs), it is now
static.

Submitted by:   kensmith