AP_Winch: fix Daiwa reporting and comments re clutch engaged

[rmackay9]

Bug Report

As discussed in this mavlink PR mavlink/mavlink#1781, the AP_Winch_Daiwa library misinterprets the meaning of the clutch being engaged vs disengaged (we have it backwards).

We also need to:

1. cherry-pick the above linked mavlink PR into the ArduPilot mavlink repo
2. update the ArduPilot mavlink submodule so that the flight code uses the updated mavlink repo
3. update the WinchData struct's clutch variable comments to be something like "0:clutch engaged (strong), 1:clutch engaged (weak), 2:clutch disengaged , motor can spin freely"
4. update the AP_Winch_Daiwa::update_user() method so the wording is consistent with the mavlink spec (e.g. "disengaged", "engaged (weak)" and "engaged (strong)")
5. update AP_Winch_Daiwa::send_status() so that it uses MAV_WINCH_STATUS_CLUTCH_DISENGAGED (instead of MAV_WINCH_STATUS_CLUTCH_ENGAGED)

Platform
[ ] All
[ ] AntennaTracker
[x] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

[peterbarker]

PR against ArduPilot mavlink is here: ArduPilot/mavlink#519