Support Voltronic Power P15/P16 inverters/SCDs

[zykh]

These devices use a protocol (https://github.com/zykh/nut-website/blob/voltronic-sunny/protocols/voltronic-sunny.txt) which is an evolution of the one used by Voltronic Power UPSes and, as such, nutdrv_qx is the best candidate to support them. With @nickma82, we've put together a subdriver (https://github.com/zykh/nut/tree/nutdrv_qx_voltronic-sunny_rebased+command) which works smoothly (as far as I can remember): @nickma82 has been successfully using it for the last several (3? 4? -- I can't remember) years. Mainly (apart from other nutdrv_qx minor issues I'm already working on), it only needs to be adapted to #513 and to get some, related, small tweaks.