?&char str_tok(?&char s, const &char delim, &?&char saveptr)Reentrant tokeniser (wraps strtok_r / strtok_s).
On first call: pass the string to tokenise as s and a '?&char' cursor
for saveptr. Subsequent calls: pass null as s.
Returns the next token, or null when exhausted.
The input string IS modified (NUL-terminators are written).