add crash detection, allow disengage via param CRASH_DETECT
improved is_flying behavior
take off, landing and hard-landing improvements
add stillness check to is_flying and log it
minimum airspeed is determined ARSPD_FBW_MIN*0.75
this fixes a bug where a mode switch change during an AUTO mission
which does not change the flight mode would cause cross tracking to be
reset, so the plane will not correctly follow the desired track
Many thanks to Michael Du Breuil for the log that showed this bug
setup the MODESWITCH channel as our OVERRIDE_CHAN, allowing for
instant manual passthru in px4io when OVERRIDE_CHAN goes above 1750
This makes for faster switching, and avoids bugs where a still enabled
FMU channel disables override
When arming we need to ensure that we don't enable any channels
otherwise PX4IO will think that FMU is active and will stop running
the internal RC mixer