add RestartGamemode command

This allows gamemode to leave the context and then enter it again, which
re-applies all system optimizations. It is useful in cases where another
program (like TLP) may override gamemode's optimizations.

This is exposed to users by the -R or --restart flags to gamemoded.

daemon/gamemode.h

@@ -155,6 +155,16 @@ int game_mode_context_unregister(GameModeContext *self, pid_t pid, pid_t request
  */
 int game_mode_context_query_status(GameModeContext *self, pid_t pid, pid_t requester);
 
+/**
+ * Restart gamemode if it is running
+ *
+ * @param pid Process ID for the remote client
+ * @returns 0 if gamemode was restarted
+ *          1 if gamemode was already deactivated
+ *          -2 if this request was rejected
+ */
+int game_mode_context_restart(GameModeContext *self, pid_t pid, pid_t requester);
+
 /**
  * Query the config of a gamemode context
  *