fix commander: handle mode executor correctly on disarm

There were a number of cases where the state was not correct or not as
desired after disarming, when running an external mode 'MyMission' with
executor:
- run MyMission, which triggers Hold, then Land
  - before: Mode: Hold, executor_in_charge: 1
  - after:  Mode: MyMission, executor_in_charge: 1
- run MyMission, then user switches to RTL
  - before: Mode: MyMission, executor_in_charge: 0
  - after:  Mode: MyMission, executor_in_charge: 1
- run MyMission, then while in Hold mode, low battery failsafe (RTL)
  - before: Mode: Hold, executor_in_charge: 1
  - after:  Mode: MyMission, executor_in_charge: 1
- run MyMission, then stop external mode (terminate the process)
  - before: Mode: (mode not available), executor_in_charge: 0
  - after:  Mode: Hold, executor_in_charge: 0

This case is unchanged:
- run MyMission, then low battery failsafe (RTL)
  - before: Mode: MyMission, executor_in_charge: 1
  - after:  Mode: MyMission, executor_in_charge: 1

src/modules/commander/UserModeIntention.hpp

@@ -53,6 +53,8 @@ public:
 	 * @return nav_state or the mode that nav_state replaces
 	 */
 	virtual uint8_t getReplacedModeIfAny(uint8_t nav_state) = 0;
+
+	virtual uint8_t onDisarm(uint8_t stored_nav_state) = 0;
 };