Add a seatbelt to IDN

From paf on dns-operations:

Please please please do check what happens when you have bidirectional strings before you decide that having U-LABEL output be the default.

I am the conservative kind that am so nervous over these kind of things that I would say "let the user turn on IDN output if the user know what the user is doing".

You might require LOCALE processing, and get different result depending on the shell you use, so "just" look at whether the output is a TTY is something that I think is not enough.

So be careful, and do proper QA, by at least reaching out to people having different directionality by default.

For bidi issues, please look for example on these old blog posts of mine:

https://stupid.domain.name/node/681 https://stupid.domain.name/node/682 https://stupid.domain.name/node/683

You might at least make your code more stable. Add a seatbelt...