lib: Implement str_sanitize_utf8().

lib: Implement str_sanitize_utf8().

Unlike str_sanitize(), this function truncates strings based on a UTF8 code point
limit rather than a maximum size in bytes. Also, the Unicode replacement
character is used to mark invalid/control characters and an ellipsis character
is used to indicate the string truncation. For the normal str_sanitize() this is
done using a question mark and triple dots respectively.