Add NUT daemon options to refuse insecure startup

[jimklimov]

Currently NUT daemons simply warn about world-readable files with potentially secret data (like upsd.users or upsmon.conf with clear-text passwords, or driver-server socket files).

This proposal is to add an option to enable/disable (eventually enable by default?) aborting daemon startup until such security issues are addressed.

• Files with passwords, like those above and nutauth.conf from Feature request: more NUT clients should be SSL-capable with proper certificate trust and all #3329
• Eventually this should probably include private key files for SSL (although they are protected with a passphrase... stored in NUT config files).
• Involve strictness levels, e.g. to also check that no configs/scripts/... are writable by the run-time user account (not only files relevant for the daemon, but all of them since different daemons may run as different accounts)?
• Perhaps make some of it a separate pre-flight check program/script that SMF/systemd methods or init scripts can invoke...

Loosely related to #3411 (refusing anonymous reads), and to #3331 and #3329 (as far as requiring that everyone talks SSL, at least when built with NSS or OpenSSL backends that are now equally capable).