waylandloginmanager: only allow the FIFO reader to send the first command to the loginmanager_listener, so that users can't flood it with multiple commands on one request such as
echo "Login
Login
Login
Login" > /run/waylandloginmanager/loginmanager_control

git-svn-id: https://rebeccablackos.svn.sourceforge.net/svnroot/rebeccablackos@2969 56e3d1a8-0179-4092-b3e2-9fbbcda0c0a9