version: unexport all vars, turn Short/Long into funcs

The other formerly exported values aren't used outside the package,
so just unexport them.

Signed-off-by: David Anderson <danderson@tailscale.com>

derp/derp_server.go

@@ -1736,7 +1736,7 @@ func (s *Server) ExpVar() expvar.Var {
 	}))
 	m.Set("counter_tcp_rtt", &s.tcpRtt)
 	var expvarVersion expvar.String
-	expvarVersion.Set(version.Long)
+	expvarVersion.Set(version.Long())
 	m.Set("version", &expvarVersion)
 	return m
 }