Plane: added FLIGHT_OPTIONS bit 4 for THR_MIN behaviour

[tridge]

This option enables use of THR_MIN in all modes when armed. For motors that need a min throttle to stay running (eg. some turbines) we need to make sure we don't drop to zero on initial failsafe

[Hwurzburg]

@tridge forget about this one?

[peterbarker]

Yep, definitely forgotten :-)

We actually have a modification in place for turbines already, but this looks rather more impactful a change.

I'm sure this must interact with ICE_IDLE_PCT - somehow. @IamPete1

[IamPete1]

There have been quite a bit of restructuring in the throttle handling. I think the ICE idle stuff would work, but it may not be a perfect fit.