MAVProxy ignores --mav20

Hello,
When I run SITL using
`sudo python sim_vehicle.py -v ArduCopter -m "--mav20" -f hexa --console --map -L WIPB`
it should use MAVlink 2.0 Protocol
As i Can assume "-m" means additional mavproxy arguments

Despite of using --mav20 when i run some C code to read data i can see

```
Bytes Received: 34
Datagram: fe 1a a5 01 01 1b e4 81 6e 6e 00 00 00 00 00 00 00 00 17 fc 02 00 02 00 01 00 b7 00 37 00 c2 01 e9 b1
Received packet: SYS: 1, COMP: 1, LEN: 26, MSG ID: 27
```

It looks like MAVlink 1.0== it starts with 0xfe

When i read data from QGroundControl it looks like
```
Bytes Received: 21
Datagram: fd 09 00 00 16 ff 00 00 00 00 00 00 00 00 06 08 c0 04 03 87 fa
Received packet: SYS: 255, COMP: 0, LEN: 9, MSG ID: 0
```
Byte array starts with 0xfd so it is MAVlink2.0

Why SITL ignore "--ma20" commands ? What should change to use MAVlink 2.0 version?

Start with sim_vehicle.py's handling of the -m argument and the MAVProxy --mav20 option. Compare the emitted packet prefixes with the reported 0xfe and 0xfd examples; done when the documented invocation produces MAVLink 2 packets as expected.

Written by the indexing model from the issue text.