Rover: proximity pre-arm check should only check forward and back

Rover-3.3 includes object avoidance which is great but the [proximity sensor pre-arm check](https://github.com/ArduPilot/ardupilot/blob/master/APMrover2/AP_Arming.cpp#L107) checks for anything within 60cm.

First off, we shouldn't use a hard-coded 60cm distance. Instead it should use the Avoidance margin parameter ([declared here](https://github.com/ArduPilot/ardupilot/blob/master/libraries/AC_Avoidance/AC_Avoid.h#L139)) instead.

Also we should only check forward and backwards.

Start in APMrover2/AP_Arming.cpp at the proximity sensor pre-arm check around line 107, then read the avoidance margin declaration in libraries/AC_Avoidance/AC_Avoid.h. Update the check to use that margin and limit it to forward and backward directions; done means the pre-arm check no longer uses a fixed 60 cm distance or checks other directions.

Written by the indexing model from the issue text.