AP_HAL: add force_safety_on method
HAL_PX4: implement force_safety_on
HAL_VRBrain: implement force_safety_on
AC_WPNav: WP_SPEED_DN parameter range to 0~500

Previous permissible descent speed of 10m/s was unnecessarily lenient.
Users can still bypass the suggested range through the MP's full
parameter list if they really want a very high descent speed.
Frame_params: 3DR_Aero_RTF.param Changed tuning
Frame_params: added 3DR_AERO_M.param
Copter: reset battery_fs after dis/rearming
Copter: define limit for baro vs inav alt disparity
Copter: typo fix for baro vs inav alt disparity definition
AC_WPNav: add loiter_soften_for_landing method

This resets the position target to the current location.
Copter: soften loiter target when maybe landed

Applies to auto's land, land, loiter, pos hold and rtl flight modes
Copter: add land_complete_maybe flag
Copter: rename land_maybe_complete function
AP_Motors: throttle_pass_through accepts pwm
Copter: move esc calibration to separate file

Restructured into case statement
Replaced use of g.throttle_max with definition
Added more comments
Send message to ground station instead of printing on console (although
probably both are unlikely to be read)
Copter: auto esc calibration

Throttle is raised to maximum and then lowered without any input from
pilot