Allow user to level vehicle by sending PREFLIGHT_CALIBRATION with param5=2

AC3.3 and some recent version of Plane supports leveling the accelerometers to the vehicle's current attitude if the PREFLIGHT_CALIBRATION message is sent with param5 = 2.0.

Code in Copter:
https://github.com/diydrones/ardupilot/blob/master/ArduCopter/GCS_Mavlink.cpp#L1264

Closed issue in ardupilot to add this support for the vehicles:
https://github.com/diydrones/ardupilot/issues/1856

Commit from Tridge that added this support to copter:
https://github.com/diydrones/ardupilot/commit/aac652a1b40916612e6895e3e61b0e38d0abd879

Start by reading the linked ArduCopter GCS_Mavlink.cpp implementation and the referenced ArduPilot issue and commit to understand the PREFLIGHT_CALIBRATION param5=2 behavior. Then locate Mission Planner's existing PREFLIGHT_CALIBRATION command path and verify that users can send param5=2.0 for vehicle leveling; done means the command is supported without breaking existing calibration actions.

Written by the indexing model from the issue text.