Move rudder arming up into RC_Channel library

[peterbarker]

This will make rudder-arming consistent across our vehicles.

One known behaviour change is that the rover arm interval has moved from 2 to 3 seconds (to be like Copter and Plane).

Other known changes in behaviour:

• Copter and Rover will try to arm if your throttle is non-zero, and you will get a warning that your throttle is non-zero. In master we don't try to arm. I think this new behaviour is far preferable from a user's standpoint as they're told why they can't arm right now in the case they meant to arm! The new code has the same behaviour as Plane in master

We can't move the rudder-specific arming checks into the RC_Channels library as ToyMode also arms with a reason of "rudder" and presumably wants the same checks

[peterbarker]

Tested in SITL - standard arming and RUDDER_DISARM values.

[tridge]

@rmackay9 also noted on the call that he's happy for the 10s trim feature to be removed if whoever removes it ensures docs are updated

[rmackay9]

Docs for autotrim are here: https://ardupilot.org/copter/docs/autotrim.html and here: https://ardupilot.org/copter/docs/arming_the_motors.html. The 2nd link in particular has a warning that could be updated to be clear it only applies to 4.0 (and earlier)