ipn: use *Prefs rather than Prefs throughout.

Prefs has become a heavy object with non-memcpy copy
semantics. We should not pass such a thing by value.

Signed-off-by: David Anderson <dave@natulte.net>

ipn/message.go

@@ -24,7 +24,7 @@ type StartArgs struct {
 }
 
 type SetPrefsArgs struct {
-	New Prefs
+	New *Prefs
 }
 
 type FakeExpireAfterArgs struct {
@@ -191,7 +191,7 @@ func (bc *BackendClient) Logout() {
 	bc.send(Command{Logout: &NoArgs{}})
 }
 
-func (bc *BackendClient) SetPrefs(new Prefs) {
+func (bc *BackendClient) SetPrefs(new *Prefs) {
 	bc.send(Command{SetPrefs: &SetPrefsArgs{New: new}})
 }