Largely removes the sudden jerk that occurs when the user pushes the
safety switch

Also changed mode_allows_arming function to accept arming_from_gcs param
Also remove AUTOTUNE from arming list

Fixes issue in which user only had 5 seconds after starting auto-trim to
raise the throttle before the auto-disarm would kick-in.

this prevents a floating point error caused by using an uninitialised
vector3 when switching between DCM and EKF control in AP_InertialNav

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>

report we have flow control on UDP and TCP

This returns true if the gyros have been calibrated successfully

the main issue was the use of the last throttle in the throttle slew
rate control, but manual skid steering was also broken

Also initialise throttle_zero  to true on startup
Treat throttle less than zero as zero

this decodes DSM using the RCIN pulses from the PRU