string-util: add strextendf() helper, that allows extending some allocated string...

string-util: add strextendf() helper, that allows extending some allocated string via a format string

It's not going to be efficient if called in inner loops, but it's oh so
handy, and we have some code that does this:

   asprintf(&p, "%s…", b, …);
   free(b);
   b = TAKE_PTR(p);

which can now be replaced by the quicker and easier to read:

   strextendf(&p, "…", …);