C-Netz: Add tiny database to track if phones are online

Reject calls, if not online.

Use an interval to check if the phones are still online.

src/cnetz/main.c

@@ -32,6 +32,7 @@
 #include "../common/freiton.h"
 #include "../common/besetztton.h"
 #include "cnetz.h"
+#include "database.h"
 #include "sysinfo.h"
 #include "dsp.h"
 #include "telegramm.h"
@@ -292,6 +293,8 @@ fail:
 	if (use_mncc_sock)
 		mncc_exit();
 
+	flush_db();
+
 	/* destroy transceiver instance */
 	while (sender_head)
 		cnetz_destroy(sender_head);