Optimise reading large numbers of --server options. When re-reading
upstream servers from /etc/resolv.conf or other sources that
can change dnsmasq tries to avoid memory fragmentation by re-using
existing records that are being re-read unchanged. This involves
seaching all the server records for each new one installed.
During startup this search is pointless, and can cause long
start times with thousands of --server options because the work
needed is O(n^2). Handle this case more intelligently.
Thanks to Ye Zhou for spotting the problem and an initial patch.

If we detect that a DNS reply from upstream is malformed don't
return it to the requestor; send a SEVFAIL rcode instead.