AP_RCProtocol: move handling of throttle-failsafe-value up to AP_RCProtocol

[peterbarker]

• will make all of the bindtime-value-throttle-failsafe-handling code common between the vehicles
• will make the RC library stop providing any values when the throttle failsafe triggers
• read() on the library can start returning false when the throttle value goes below the threshold value meaning no other layer needs to know how the RC receiver is telling us it is unhappy (which can be protocol bit, bind-time values or simply no pulses!)

Still need to do vehicles-other-than-Copter.

[peterbarker]

Current formulation has the consequence that the low throttle value causing a low-throttle-value failsafe is never seen via mavlink because we return 'false' in AP_RCProtocol::read now for this situation, so the internal state of RC_Channels is no longer updated. Same is true of the other channels, of course.

[andyp1per]

I mean LGTM, but it will require a decent amount of testing since here be dragons...

[andyp1per]

Let's put the #define back in - makes it easier to review

[peterbarker]

I mean LGTM, but it will require a decent amount of testing since here be dragons...

Yep! This is really a rough-cut at the moment to work out whether people are happy with the layering etc etc.

I don't think I'll be pushing this even for 4.7 - it's just too critical.

Another option instead of the call would be to create parameters for the AP_RCProtocol library - which would also fix up the nasty hackery where it "borrows" parameters from RC_Channel!