Improved UAVCAN status reporting

```
nsh> uavcan status
ESC actuators control groups: sub: 1 / req: 1 / fds: 2
ESC mixer: OK
Sensor 'gnss':
RX errors: 0, receiver node id: 50

Sensor 'mag':
devname: /dev/mag
channel 0: node id 50 --> class instance 1
channel 1: empty
channel 2: empty

Sensor 'baro':
devname: /dev/baro
channel 0: node id 50 --> class instance 1
channel 1: empty
```