Verify_takeoff was always returning false if throttle was zero.  The
pilot should be able to move their throttle back to zero immediately
after initiating the mission

the choice of analog source is now in the airspeed driver

this gives us support for arbitrary analog pins for the airspeed
sensor, plus support for the EagleTree airspeed driver on PX4

This bug affected parameters where the defaults are overridden in the
object constructor. For example, a PID object may have a default value
for PID_D of 0.0, but have a constructor based default of 0.2. If the
user tries to set the value to exactly 0.0, then the set wouldn't happen,
as the value matches the value in the object default var_info[]
table. 

This change ensures we force a save to eeprom if the value is changing
from the current value, regardless of the var_info[] default.

Thanks to Tom Coyle for finding this bug!

just needs a wrapper driver in APM now

Nose of copter now points at next guided point if it is more than 10m
away

if the user zeros the integrator tuning factor, we need to zero the
integrator

gives some chance of knowing if it worked

not all pins were defaulting to this, which led to some strange
readings on some devices. First noticed by Andi on a LM335 temperature
sensor