lights_helper: I/O API cleanup

 * buf => buffer for consistency
 * explicitly cast strtol retval to int
 * close(NULL) is a no-op, remove superfluous checking
 * replace variying buffer sizes by INT_MAX_STRLEN
 * always return signed errno codes

Change-Id: I972e33e2435ec7d68a8ffdc5ab88a14d8005ff55