AP_Mission: added MIS_AUTORESTART parameter

- added MIS_AUTORESTART parameter, defaults to 0
- added start_or_resume() function to either start or resume a mission
based on that parameter value
AP_Mission: added reset() function
Plane: RC override input should be considered as valid RC input

this prevents us setting trim values under joystick control
Plane: use MIS_AUTORESET when entering AUTO mode
AP_Mission: improve the AP_Mission docs

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