Fix for Spring loaded throttle in manual throttle modes

This is a patch which introduces a throttle control for disengaging throttle when arming via mavlink while in a manual mode such as Acro or Stabilize. 

All other modes with alt hold don't need to check throttle position so this removes the "Arm: Throttle too high" check. 

UX : user enters stabilize mode, presses ARM on Droidplanner. motors spool up to idle and the user brings throttle to zero. Throttle engages at zero and as it is raised it works normally.