Mission: bug fix set_current_cmd

If mission had completed desired command index was ignored
Plane: bug fix to set_current_cmd

When set_current_cmd mavlink message is received, resume mission only if
the mission has completed.
Stop mission when vehicle leaves AUTO mode.
Plane: rename next_nav_command to auto_rtl_command

This command is only used to get the vehicle home (or to a rally point)
after the mission has completed