Feature request: Support instcmds for nut-upsd

[dmcc]

I was wondering if we could have it so that setting $INSTCMDS would be passed to upsd.users. My specific use case is to make it possible to temporarily mute the UPS beeper, but others (e.g., running tests) might be useful as well.

If this is of interest, please let me know and I'll put together a PR.

[dmcc]

Alternatively, if making a local config in /etc/nut/local is preferred, I can understand that -- I'm basically wondering if instcmds is a common enough override.