Copter: add pitch axis flipping

Added Pitch axis flipping. You must be pushing forward or back 3° to make it flip in Pitch axis or it will flip in the default roll axis.
Added some extra throttle control to maintain altitude
removed the CH7 ability to exit Flip mode. It will exit only if completing flip or user aborts with control sticks.
Copter: flip minor comments and formatting
Copter: remove unused flip_stop
Copter: ensure motors don't stop during flip
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