string-util: when ellipsizing to a length if (size_t) -1, become a NOP

Let's say that (size_t) -1 (i.e. SIZE_T_MAX) is equivalent to
"unbounded" ellipsation, i.e. ellipsation as NOP. In which case the
relevant functions become little more than strdup()/strndup().

This is useful to simplify caller code in case we want to turn off
ellipsation in certain code paths with minimal caller-side handling for
this.