`sdlog2 start` lead to a segfault.

Only free the logbuffer if you exit sdlog2. After disarming, just reset
the pointers. This way, we don't take any risks doing malloc and free.

The actual "double free" was caused by trying to free the perf counter
inside logbuffer_free.

This allows resetting of the logbuffer without having to free and malloc
again.

added descend into navigation using existing land helper, downstream logic handles ignoring position setpoints (despite being populated), tested with multirotor in gazebo

Timestamps didn't exist when commander was run on the Linux side.
However, commander is now always run on the QURT side, so the hack is no
longer needed.

PWM output is now stopped if the mavlink actuator_controls stop being
sent. This means props will stop in case the Snapdragon crashes or the
cable from Snapdragon to Pixhawk/Pixracer breaks.

The mavlink instance USB can now be shutdown, so this hack is not needed
anymore.

This includes taking care of the arming state and the limit ramp.

This is by far the easiest solution for now.