[server] add airspeed data in FLIGHT_PARAM message
[server] send negative value if no airspeed available

on a second thought it is safer to tell that there is no airspeed data,
ground speed is already available in the message anyway
[server] change again airspeed field position

for compatibility with PPRZonDroid beta version
[rotorcraft] rc setpoint handling refactored

closes #684 which partially solves #599