auth: Send uid rather than username when authenticating

There is no particular advantage to sending the username. It just
means the other end has to map it to a uid when comparing with the
uid from getpeercred. Additionally that mapping is not always
possible, as the remote side may not have the correct /etc/passwd.
For instance, this can happen when talking to a dbus service inside
a container.

auth.go

@@ -5,7 +5,9 @@ import (
 	"bytes"
 	"errors"
 	"io"
+	"os"
 	"os/user"
+	"strconv"
 )
 
 // AuthStatus represents the Status of an authentication mechanism.
@@ -52,11 +54,12 @@ type Auth interface {
 // bus. Auth must not be called on shared connections.
 func (conn *Conn) Auth(methods []Auth) error {
 	if methods == nil {
+		uid := strconv.Itoa(os.Getuid())
 		u, err := user.Current()
 		if err != nil {
 			return err
 		}
-		methods = []Auth{AuthExternal(u.Username), AuthCookieSha1(u.Username, u.HomeDir)}
+		methods = []Auth{AuthExternal(uid), AuthCookieSha1(uid, u.HomeDir)}
 	}
 	in := bufio.NewReader(conn.transport)
 	err := conn.transport.SendNullByte()