sbus1_output

Two patches for sbus1_output in sbus.c.
The first implements sbus1_output for all channels except the single bit digital ones. This patch has been flown underneath ArduPlane.
The second patch cleans things up. My late night algebra was a little out with the rounding error .5f and I wasn't happy with the safety of the bounds checking. I'll hopefully get to fly this in the morning.
It would be preferable to allocate this on the stack (in the function) to save global RAM. The input frame is only allocated globally because it is filled byte by byte.