dns-example: allow to set ns from args

We can't do this using resolv.conf:
$ dns-example -v -c <(echo nameserver 127.0.0.1:10053) ya.ru
Because of how evutil_read_file_() works (using fstat())

And actually glibc for example will not use port from nameserver line, and
because of inet_aton() it will fail if nameserver will have arbitary port:
(gdb) p inet_aton("127.0.0.1", malloc(10000))
$1 = 1
(gdb) p inet_aton("127.0.0.1:53", malloc(10000))
$2 = 0

From glibc/resolv/res_init.c:
  if (MATCH(buf, "nameserver") && nserv < MAXNS) {
      struct in_addr a;
      cp = buf + sizeof("nameserver") - 1;
      while (*cp == ' ' || *cp == '\t')
          cp++;
      if ((*cp != '\0') && (*cp != '\n')
          && __inet_aton(cp, &a)) {
          statp->nsaddr_list[nservall].sin_addr = a;
          statp->nsaddr_list[nservall].sin_family = AF_INET;
          statp->nsaddr_list[nservall].sin_port =
                  htons(NAMESERVER_PORT);