[th/strbuf-printfv] glib-aux: add nm_str_buf_append_printfv()

I had a patch that needed this. In the end, that patch was dropped. But in general, a variadic version of a function (printf()) almost always also needs a va_list variant (vprintf()).

Add it.

Since we can implement nm_str_buf_append_printf() based on nm_str_buf_append_printfv(), the code is still used.