AP_TECS : Prevent throttle undershoot after climb

If the plane was unable to achieve the climb and got significantly below the internal TECS demanded climb profile, the the PD term would be a large value at the top of climb, and would take some time to reduce due to the rate limiter. This meant that the integrator state could be pushed to a very low value and effectively cause the throttle to sit on the lower limit for longer than desired after levelling out.
AP_TECS: fixed throttle demand on underspeed

thanks to Philipp Oettershagen for finding this bug!
AutoTest: fix fly_mission to recognise end of mission

Copter's behaviour after completing a mission has changed so it does not
automatically switch to Loiter mode but rather stays at the last
waypoint.  This was confusing the fly_mission function.
HAL_PX4: use px4io generated values for servo output readback

this makes it easier to observe the behaviour of IO failsafe
Copter: remove abbreviations from GCS messages
Copter: allow starting autotune from flightmode switch

ch5 switch to AutoTune is equivalent to starting autotune via sw7/8.
Copter: minor reformat of control_autotune
build_devrelease: build all frames for jons release
AP_Mount: Added lead filter
DataFlash: add UBX3 message for accuracy logging
AP_GPS: Add uBlox accuracy metrics interface and logging
AP_NavEKF: Add parameter that specifies EKF-to-DCM/INAV fallback strictness
AP_Arming: remove abbreviations from GCS messages
Copter: ensure lat, lon, alt all zero is handled

Use default position when waypoint lat/lon are zero,
and default altitude when waypoint alt is zero, for
both spline and straight-line waypoints.

Fixes #1145.
AutoTest: copter test that zero alt defaults to current alt
AC_WPNav: minor fix to comments
Copter: clarify pv_location_to_vector param name

Also update function description
Plane: add support for generating a PX4 mixer for failsafe

this creates APM/MIXER.MIX which will be used if the FMU dies to
provide manual control over RC
Plane: added OVERRIDE_CHAN parameter

this allows both for testing PX4IO override on the ground, and for
forced manual override while flying, including re-arming after in-air
reboot
RC_Channel: added channel_function()

this is used in the plane mixing code