[client] keybind: add ScrollLock+C to cycle microphone defaults

This makes it possible to change the default action taken the next time
an application tries to open the microphone without restarting the
client.

client/src/main.h

@@ -50,6 +50,7 @@ enum MicDefaultState {
   MIC_DEFAULT_ALLOW,
   MIC_DEFAULT_DENY
 };
+#define MIC_DEFAULT_MAX (MIC_DEFAULT_DENY + 1)
 
 struct AppState
 {