AP_SpdHgtControl: added height above field parameter
TECS: use altitude supplied by mainline code

this allows for use of ALT_OFFSET and ALT_MIX
Plane: pass in adjusted height above field to TECS
TECS: added support for throttle nudging
Plane: added support for throttle nudging with TECS
AP_Param: use 0xFFFF not __UINT16_MAX__

MacOS doesn't have __UINT16_MAX__