Also flush prefs on user change, see previous commit

Flushing user changes to powder.pref was in fact the reason why I began this pref flushing thing at all, but of course Client handles user changes by modifying preferences directly, not through SetPref, so it skipped WritePrefs.

src/client/Client.cpp

@@ -940,6 +940,7 @@ Client::~Client()
 void Client::SetAuthUser(User user)
 {
 	authUser = user;
+	WritePrefs();
 	notifyAuthUserChanged();
 }