Add helpers to truncate UTF-8 at code points

This will help to avoid truncating a UTF-8 string in the middle of a
code point, producing an invalid UTF-8 result.

app/src/str_util.h

@@ -23,6 +23,10 @@ xstrjoin(char *dst, const char *const tokens[], char sep, size_t n);
 char *
 strquote(const char *src);
 
+// return the index to truncate a UTF-8 string at a valid position
+size_t
+utf8_truncation_index(const char *utf8, size_t max_len);
+
 #ifdef _WIN32
 // convert a UTF-8 string to a wchar_t string
 // returns the new allocated string, to be freed by the caller