Add visual or audible alert support on connect/disconnect

The feature is detailed via the following option:

 --alert none|bell|blink

     Set alert action on connect/disconnect.

     It will sound the bell once or blink once on successful connect.
     Likewise it will sound the bell twice or blink twice on disconnect.

     Default value is "none" for no alert.

src/alert.h

@@ -0,0 +1,33 @@
+/*
+ * tio - a simple serial terminal I/O tool
+ *
+ * Copyright (c) 2014-2022  Martin Lund
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ */
+
+#pragma once
+
+enum alert_t
+{
+    ALERT_NONE,
+    ALERT_BELL,
+    ALERT_BLINK,
+    ALERT_END,
+};
+
+void alert_connect(void);
+void alert_disconnect(void);