multi: handle timeouts on DNS servers by checking for new sockets

If the first name server is not available, the multi interface does
not invoke the socket_cb when the DNS request to the first name server
timesout.  Ensure that the list of sockets are always updated after
calling Curl_resolver_is_resolved.

This bug can be reproduced if Curl is complied with --enable_ares and
your code uses the multi socket interfaces and the
CURLMOPT_SOCKETFUNCTION option.  To test try:
  iptables -I INPUT \
           -s $(sed -n -e '/name/{s/.* //p;q}' /etc/resolv.conf)/32 \
           -j REJECT
and then run a program which uses the multi-interface.