Warn for CYRILLIC, unknown

[thinkyhead]

Allow builds not using HD44780 that happen to have weird DISPLAY_CHARSET_HD44780 values to pass.

Also suppress deprecated sprintf warnings to use snprintf that should instead be warning us to use Rust.

[p3p]

I don't mind cosmetic PRs, although some of the changes I don't agree with, but not merged with real fixes putting off rebasing this to remove them is what has delayed merging ^^

[thinkyhead]

Well, what I do with my projects is to cherry-pick the pieces I like from a PR, trying to preserve author information in the final result. One simple way to get there is to do a git diff HEAD master | git apply to revert all changes without any commit, then construct a commit with only selected changes reverted, the rest left as-is. Sometimes I do a bit of extra labor to make sure nothing is missed. I don't like to push this stuff back on the contributor because it's usually just faster to do it myself and get it exactly right in one go.