- removing unused variables
- removing unused code blocks
- improving code readability
- initialising members at definition

An airframe change keeps all RC params except the flight modes. This fixes
that.

 - this is needed for the unit tests

 - add preflight_check helper

 - this flag often results in hiding useful information, or adding
   useless information to the mavlink console

 - only call prearm if preflight passes
 - prearm always provide feedback

 -  fixes #9155

Battery warning gets only fired if a battery is inserted.

The goal is to improve the readability for expressions like:
if (hrt_elapsed_time(&start_time) > 8000000)

Available since C++11