[RFC] replace non utf8 always with the same char

[Artem Kachitchkine]

> Yes, it would probably be good to have a shared library that all helpers
> can link statically [1] for all backends. It should probably live in
> hald/helper_utils. 
> 
> Joe, Artem, Danny, Richard, what do you think?
> 

Sounds good to me.

-Artem.