Copter: added set_nav_mode to control initialisation of nav controllers

Renamed run_navigation_controllers() to run_autopilot()
Renamed update_nav_wp() to update_nav_mode()
Renamed wp_control to nav_mode to be more consistent with roll-pitch,
yaw and throttle controllers
Copter: leonard's inav2

Moved loiter controller to navigation.pde
Copter: set each flight mode's nav_mode

Allow ACRO flight mode's roll, pitch, yaw, throttle and nav mode to be
overwritten