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.
To maximize efficiency, it is better to use the native int time for iterators:
```
for (unsigned i = 0; (i < num_values) && (i < 16); ++i)
```
Non-native types can actually force additional operations, and the registers anyway are 32bit wide.