ArduCopter: bug fix to ppm encoder failsafe

A new radio frame could arrive just as the frame time vs current time
check was being executed causing a false positive
AP_InertialSensor: return roll and pitch trim angles
Copter: integrate automatic roll and pitch trims
Copter: bug fix to set sonar_alt_health to zero when disabled
Copter: bug fix for autotrim's roll axis backwards
Copter: THR_MID used to scale manual throttle
Copter: update firmware version to 2.9.1-rc1

Release Notes updated
Copter: use scaled throttle for accel-throttle's I term

Accel throttle's I term is taken from scaled manual throttle
Copter: use home position for scaling Longitude
Copter: manual throttle scaling fix

Scaling of bottom half of throttle was changed from THR_MIN ~ 500.
Previously it was from 0 ~ 500.
Copter: update release version to 2.9.1-rc2
Copter: set update rate to 50hz during esc calibration

Also modified some comments in the code re the esc calibration
Copter: updated release notes for 2.9.1-rc2
Copter: update version to 2.9.1 ahead of release

Also updated release notes
Copter: limit max ahrs trim to 10degrees

Limit is applied as trims from accelerometer calibration are copied to
the ahrs object.