config: Allow renice configuration

This commit adds configuration support for the renice value and amends
documentation and examples. This commit by itself does nothing, the
following commit is needed to actually apply the new settings.

Signed-off-by: Kai Krakow <kai@kaishome.de>

daemon/daemon_config.h

@@ -108,3 +108,8 @@ void config_get_desired_governor(GameModeConfig *self, char governor[CONFIG_VALU
  * Get the chosen soft realtime behavior
  */
 void config_get_soft_realtime(GameModeConfig *self, char softrealtime[CONFIG_VALUE_MAX]);
+
+/*
+ * Get the renice value
+ */
+void config_get_renice_value(GameModeConfig *self, long *value);