Improve support for SCDs

[zykh]

We currently have an initial limited support for some SCDs (driver ivtscd), but the retrieved and published data is very scarce. Other (newer) devices report more data which could be useful to know (e.g. https://github.com/zykh/nut-website/blob/voltronic-sunny/protocols/voltronic-sunny.txt), but we don't have yet NUT names to map these things. Also, some devices lack the 'UPS-part' and, if I recall correctly, this cannot be avoided in NUT (e.g. there's always an ups.status).

So, at least, we should:

• [ ] add new NUT names for SCD-related data,
• [ ] (possibly) make NUT less UPS-dependent.

Other?