this also fixes issue #292

This will make stops less aggressive when entering loiter at very high
speeds (more 7.5m/s)

Speed of intermediate point is initialised and constrained based on
current speed vector vs direction to the next waypoint.
This means the copter does not slow if the previous segment and next
segment are in line and also the intermediate point is not advanced
towards the next waypoint if the copter is moving quickly in the
opposite direction.

also adds more error checking

small formatting changes too

We should keep the order in the case statement the same as the #define
order.

Should allow a dropdown in the GCS so it can be set more easily without
going into the cli

set ARMING_CHECK parameter to zero to disable

With APHal all reads are fast

With APHal all writes are fast