printf: fix floating point buffer overflow issues

... and add a bunch of floating point printf tests
int curl_mvprintf(const char *format, va_list ap_save)
{
  return dprintf_formatf(stdout, fputc, format, ap_save);
}

int curl_mvprintf(const char *format, va_list ap_save)
{
  return dprintf_formatf(stdout, fputc, format, ap_save);
}
