Improve metasys driver as per protocol specs

[zykh]

(follow-up to #616)

We now have access to (almost) all the protocols specs for metasys-supported devices:

• networkupstools/nut-website@66d30c9
• networkupstools/nut-website@570c1e5

...and the metasys driver currently only uses a subset of the available commands (only the common ones, and not even fully).

While not all of them have a corresponding entry in NUT (and some may be considered superfluous for the average user), we should, at the very least, support (by implementing setvar() and enhancing instcmd()) the programmable delays for shutdowns (currently hardcoded in the driver) and, possibly, the various settable thresholds (voltages, frequencies, etc.) and working modes.