--nodtr doesn't prevent dtr

Starting MAVProxy with the --nodtr option doesn't appear to prevent DTR, and thus the APM reboots on USB connection.

Is there a simple way to prevent this? I am happy to branch, update and commit to the repo if given a bit of direction.

Start by locating the --nodtr option handling and the serial/USB connection path in MAVProxy, then reproduce the APM reboot when the option is used. Confirm that DTR is not asserted with --nodtr and that the APM no longer reboots on USB connection.

Written by the indexing model from the issue text.