AP_Terrain: added extrapolation based on last available data

if we run out of terrain data then extrapolate using the last
available terrain height at the AHRS position. This can be used to
cope with GCS outages over long distances where the terrain data isn't
preloaded
Plane: use terrain extrapolation

use best effort extrapolation if we run out of terrain data
AP_TECS: added TECS_PITCH_MAX and TECS_PITCH_MIN

this allows the TECS controlled pitch limits to be smaller than the
FBWA limits. It is common for a human pilot to want a bit more
discretion over pitch than you want the automatic controller to use
Plane: make STAB_PITCH_DOWN in degrees

try to avoid new centi-degrees parameters
DataFlash: added TERR log message for terrain data
AP_Terrain: added dataflash logging
Plane: use dataflash logging for terrain
AP_Mission: added last_change_time_ms() call

will be used by terrain lib for scanning waypoints
AP_Terrain: added automatic waypoint checking

check we have terrain data for all waypoints
AP_Rally: added last_change_time_ms() call
AP_Terrain: added automatic rally point checking
Plane: fix for changed AP_Terrain API
Copter: fix for changed AP_Terrain API
Plane: fixed typo in description
Rover: fixed build
VARTest: fixed build
AP_Rally: fixed build for non-standard vehicle types
CPUInfo: fixed build