Add option whether to include header when logging.

It is useful to be able to exclude the header so that the log file
can be used for realtime input to other programs such as Kst for
plotting live data from sensors.

logging.c

@@ -103,7 +103,7 @@ static void logfopen_callback(void *vctx, int mode)
         }
     }
 
-    if (ctx->state == L_OPEN) {
+    if (ctx->state == L_OPEN && conf_get_int(ctx->conf, CONF_logheader)) {
 	/* Write header line into log file. */
 	tm = ltime();
 	strftime(buf, 24, "%Y.%m.%d %H:%M:%S", &tm);