GPS: add 2D fix type
AHRS: add support for GPS fix type 2D
Copter: add support for GPS fix type 2D
Plane: add support for GPS fix type 2D
GPS: fixes after review

Ensure 3D fix before returning velocity_down
UBLOX: restore check of fix_status as part of determining 2D or 3D fix
SIRF: add missing brackets on fix type check
Rover: add support for GPS fix type 2D
Plane: replace gps fix with status

Required to ensure 3d fix is used for initialising home
UBLOX GPS: fix MSG_SOL typo

"status" was used instead of "solution"