AP_TECS: New params LAND_ARSPD and LAND_THR auto-slowing landings.

Also controlling pitch by ignoring height error during landing.
This serves to keep the nose up.
AP_TECS: Auto landing now takes throttle_nudge into account.
AP_SpdHgtControl: added get_target_airspeed() interface
Plane: use TECS get_target_airspeed()
AP_TECS: make target landing speed a float

some users may want more precise control
AP_TECS: set target airspeed, not min/max airspeed, on landing

this seems to prevent ballooning of the altitude when we are in the
landing approach, and gives a much smoother landing
Plane: fixed mission resume at arbitrary waypoints

on completion of a mission a user can ask for it to resume at any wp
Copter: fix typo to show ATUN FMT in dataflash logs
AP_NavEKF : Fixes bug that causes accel bias to diverge in static mode, preventing copter arming
AP_NavEKF : Fix bug that limits copter sensor delay compensation to 125
msec
GCS_Mavlink: return MISSION_INVALID when conversion fails

Requested by MichaelO to allow better error message to be displayed to
user when the mission upload from the GCS to vehicle fails
AntennaTracker: lots of new parameters

YAW_SLEW_TIME, PITCH_SLEW_TIME, MIN_REVERSE_TIME, START_LATITUDE,
START_LONGITUDE and STARTUP_DELAY

See the parameter docs for explanations
AntennaTracker: added STOP mode

to stop tracker moving
AntennaTracker: fixed ballerina and more accurate tracking

implement new parameter options
AntennaTracker: fixed APM2 serial speed handling

need to change based on USB mux status
AP_NavEKF : Prevent large magnetic field adaptations early in flight
AP_NavEKF : Track baro height observations pre-arm

This prevents copter from failing the pre-arm height discrepancy test
AP_Compass: avoid division by zero if we haven't received any mag reports

Otherwise, get_field() will return NaNs after once every few calls to
read() during compassmot on PX4 platforms, which causes compassmot to fail.

This is a quick hack around the deeper issue, which could be something
like the PX4 mag driver experiencing starvation and skipping mag reports,
buffer overrun or something else that causes mag reports to be dropped.
Or perhaps we should never expect in the first place that we will always
receive at least one mag report between calls to read().
AP_Compass: fixed last_update for secondary compass with primary failed

this ensures EKF and DCM will use a secondary compass if the primary
fails
Rover: report unhealthy compass if primary compass unhealthy