AP_Terrain: added checking and reporting messages

this allows the GCS to check the status of the terrain system
GCS_MAVLink: added TERRAIN_CHECK and TERRAIN_REPORT messages
GCS_MAVLink: regenerate MAVLink headers
AP_Baro: minor improvements to Baro HIL code health check
Plane: handle TERRAIN_CHECK requests
Plane: fixed txsize check for airspeed info

prevent underflow