plugin: Add private method plugin_exists() to check for HOOKs

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/private.h

@@ -40,6 +40,7 @@ int       client           (int argc, char *argv[]);
 
 void      service_monitor  (pid_t lost);
 
+int       plugin_exists    (hook_point_t no);
 void      plugin_run_hook  (hook_point_t no, void *arg);
 void      plugin_run_hooks (hook_point_t no);