Based on original work by Nils Hogberg

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this prevents us setting trim values under joystick control

- added MIS_AUTORESTART parameter, defaults to 0
- added start_or_resume() function to either start or resume a mission
based on that parameter value

Resolves issue #435
If the altitude fence is enabled, when the vehicle RTLs it will return
at the lower of the RTL_ALT and the FENCE_ALT_MAX

This resolves issue #880 in which if a GPS failsafe occurred while the
vehicle was already in LAND mode, the LAND would continue to be GPS
controlled LAND instead of pilot-controlled LAND

saves 2 bytes

This resolves issue #461 by giving the pilot a minimum of 10 seconds to
attempt to manually recover before the autopilot will attempt to retake
control to bring the copter home or land.

This ensures invalid pilot desired accelerations are cleared from the
loiter controller.  This is probably not strictly necessary because the
vehicle should switch out of Loiter and into RTL during failsafe.

Resolves issue in which vehicle would get stuck at RTL_ALT_FINAL if
failsafe occured.
Also clear out pilot acceleration from loiter controller if failsafe
occurs during the final descent.

This allows quickly clearing out the pilot desired acceleration for
loiter contoller.

This resolves an issue in which the parachute could be suddenly released
when the user enabled the parachute.  The sequence that could have
caused this bad behaviour were (1) the parachute is triggered (2) the
user disables the parachute in the 0.5sec between the trigger and the
actual release, (3) the user re-enables the parachute and the old
release time from (1) is used.

This saves 1k of flash and we are desperately low on the APM1/APM2.
Hopefully we can find savings somewhere and re-enable it by default.

This fixes the issue in which a short RTL_LOIT_TIME could cause the
vehicle's heading to be caught between it's heading when it arrived at
home and the initial armed heading.  With this fix it now waits above
home until the timer has run out AND the heading is within 2degrees of
the initial armed heading.

Note that an RTL command executed in AUTO mode will also disarm when it
lands and pilot's throttle is put to zero and no further commands will
be executed.  This is normally not an issue because missions generally
end with an RTL (instead of having the RTL in the middle) and a work
around is available in that the LAND command could be used instead of
RTL.

Resolves bug in which do-set-speed allowed reducing the speed during the
mission but not increasing it.
Slow down distance is also recalculated.
Unnecessary call to calc_wp_leash_length removed from
set_spline_origin_and_destination.

Add JulianD and SandroT and replaced tabs with spaces

Removed useless call of function
hybrid_get_wind_comp_lean_angles(hybrid.wind_comp_roll,
hybrid.wind_comp_pitch);
Removed 10Hz filter for hybrid_update_wind_comp_estimate()

when you exit auto and a takeoff is still in progress the flight stage
was not updated, which led to the throttle staying at maximum

Many thanks to Marco for finding this!

We have run out of flash on the APM2, it was a choice between Hybrid
flight mode and OF_Loiter

Hybrid flight mode costs 4.5k of flash which currently puts us over the
limit for APM1 and APM2 unless optical flow or other features are
disabled