WPNav: reduce max acceleration to 2.5m/s

Also includes bug fix to ramp up of speed of intermediate point
WPNav: add fast waypoints flag

Waypoint is considered "reached" when the intermediate point reaches the
destination
Copter: set wp to "fast" if they have no delay
AP_Compass: more efficient calculate_heading() implementation

Thanks to piersh for this improvement. See

https://github.com/diydrones/ardupilot/commit/b5f0635455957d6df65c7653e3077a6fa0737ea4#commitcomment-3171806
AP_Compass: use const on more functions and remove old calculate_heading()

the calculate_heading() based on roll/pitch is not needed anywhere
GCS_MAVLink: moved gcs_severity to library header
AP_InertialSensor: use SEVERITY_USER_RESPONSE for accel cal messages

thanks to the suggestion from Kevin
Rover: use library gcs_severity
Copter: use library gcs_severity
Plane: use library gcs_severity
VARTest: fixed build
Copter: init_rc_out in cli's compassmot setup

Bug fix to allow cli's compass motor compensation to be run from the
mission planner's terminal window
WPNav: initialise desired roll, pitch

bug-fix to resolve 1/10th second twitch when loiter or waypoint
controller are first engaged
AP_Common: make PACKED depend on architecture

this will remove lots of warnings when building with arduino
AP_GPS: use PACKED attribute instead of pragma pack
SITL: use PACKED attribute instead of pragma pack
AP_Param: use PACKED attribute in examples
SITL: use PACKED attribute in fdm structure
Copter: bug fix for RTL yaw control

Nose was only pointing home if your initial altitude was above the
RTL_ALT
HAL_PX4: include visibility.h

needed for new nuttx structure