AP_SpdHgtControl: rename flybywire_airspeed to airspeed

this variable applies to any automatic throttle mode, not just FBW
modes.

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
AP_TECS: adapt for new airspeed variable names
APM_Control: pass in aircraft parameters and expose coordinate rate offset

this will make it possible to do rate based pitch control without
having a coordinated turn, for in ACRO mode

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
VARTest: adapt for new airspeed parameter names
Plane: pass aircraft parameters to attitude controllers

also cope with rename of airspeed min/max variables

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>