MAVLink channel numbering inconsistent

## Bug Report

### Issue details

message-interval-chanX.txt uses 0 based indexing
MAVx_* (former SRx) uses 1 based indexing
SERIALx_* uses 0 based indexing (with 1 based indexing exposed)

different numbering schemes and hiding SERIAL0 (USB) configuration leads to confusion when configuring mavlink channels.

I would like to propose to shift MAVx to 0 based indexing with note that 0th channel is USB port. This way numbering will be consistent between parameters and configuration files, additionally mission planner Setup -> Serial ports will appear to have consistent 1 based numbering.

**Version**
Any

**Platform**
[x] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
Any

**Hardware type**
Any

**Logs**
Not applicable

Review the numbering used by message-interval-chanX.txt, MAVx_* parameters, and SERIALx_* parameters, then inspect the Mission Planner Setup → Serial ports presentation. Define how the USB port and channel indexes should align across these interfaces; done means the parameter and configuration-file numbering is consistent and the exposed serial-port numbering remains clear.

Written by the indexing model from the issue text.