Change media monitoring interval from 500ms to 5s

It seems excessive to hit the CDROM drive every 500ms to see if media is
there or not.  Every 5s should do fine.  (Also moved the interval to a
macro (#define MONITOR_INTERVAL) to make it more maintainable).

Ideally, we should try to use dbus for this if possible (where supported)
and not poll at all.

mythtv/libs/libmyth/mythmediamonitor.cpp

@@ -57,19 +57,20 @@ void MonitorThread::run(void)
 ////////////////////////////////////////////////////////////////////////
 // MediaMonitor
 
+#define MONITOR_INTERVAL 5000
 
 MediaMonitor* MediaMonitor::GetMediaMonitor(void)
 {
     if (c_monitor)
         return c_monitor;
 
 #ifdef USING_DARWIN_DA
-    c_monitor = new MediaMonitorDarwin(NULL, 500, true);
+    c_monitor = new MediaMonitorDarwin(NULL, MONITOR_INTERVAL, true);
 #else
   #if CONFIG_CYGWIN || defined(_WIN32)
-    c_monitor = new MediaMonitorWindows(NULL, 500, true);
+    c_monitor = new MediaMonitorWindows(NULL, MONITOR_INTERVAL, true);
   #else
-    c_monitor = new MediaMonitorUnix(NULL, 500, true);
+    c_monitor = new MediaMonitorUnix(NULL, MONITOR_INTERVAL, true);
   #endif
 #endif