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/common/debug.h

@@ -14,6 +14,7 @@
 #define DFRAME		7
 #define DCALL		8
 #define DMNCC		9
+#define DDB		10
 
 #define PDEBUG(cat, level, fmt, arg...) _printdebug(__FILE__, __FUNCTION__, __LINE__, cat, level, fmt, ## arg)
 void _printdebug(const char *file, const char *function, int line, int cat, int level, const char *fmt, ...);