Mavlink Guided mode support via COMMAND_INT

[xznhj8129]

This PR adds the beginning of handling for COMMAND_INT messages and implements setting of WP#255 with MAV_CMD_DO_REPOSITION if GCS NAV is active; i leave autipilot type as MAV_AUTOPILOT_GENERIC for minimal changes. This also implements returning of Guided mode ident if GCS NAV is active as in #10424