this converts the heli code to use the SRV_Channels output
functions. It does not change behaviour, but removes the last vehicle
type that did its own servo output calculations. This change also
fixed servo initialization conflicts.
Note that this also allows helis to be setup with more than one
channel for a particular output (eg. two separate channels for tail
servo if they are wanted). This isn't likely to be used much, but does
make heli consistent with other vehicle types
bnsgeyer
ChristopherOlson
Jacob Walser
Andrew Tridgell