Plane: added FBWA_TDRAG_CHAN parameter

this allows for testing taildragger takeoffs in FBWA mode
AP_NavEKF: make use_compass() public
AP_AHRS: use EKF use_compass() if EKF enabled

this allows magfailed status to show on console via SYS_STATUS health
bits
Plane: reset steering integrator on mode change and when not moving

this prevents an old integrator from causing problems on takeoff
AP_Terrain: fetch more terrain data around waypoints

this ensures we have data for a wide region (1km) around each waypoint
Plane: run terrain.update() more often

this provides faster checking of mission waypoints
AP_Mount.cpp - correct units in description

That should be corrected also in MP as the current code is expecting
degrees, not centidegrees.
3dr Berkeley moved - updated coords
AP_NavEKF : Clear compass fail on transition between armed and disarmed

This allows a compass that has been declared failed, possibly because of
external disturbances (eg movement of hatches, proximity of tools, etc)
to be given a second chance when the vehicle is armed.
Plane: don't do failsafe passthru with no RC input

thanks to Klrill-ka for the suggestion

fixes issue #1302
Copter: make landing detector more strict

Made more strict by requiring 50 consecutive iterations where the climb
rate is below +- 40cm/s.  Previously it was 50 cumulative.

Removed check of failsafe.radio when clearing the land flag because it
could result in the vehicle taking off if the user picked it up.
GCS_MAVLink: slow down parameter send a lot with no flow control
Plane: prepare beta4
Plane: prepare for 3.1.0 release
Rover: prepare for 2.46 release
AutoTest: reduce copter throttle when flying square

This will keep the copter lower during this test and resolve the timeout
during landing
LogAnalyzer: convert tabs to spaces (only) per coding conventions
LogAnalyzer: Report missing parameters as test failures
LogAnalyzer: If skipping badlines, ignore parse errors
Copter: remote arming check reference to compass learning