Improve handling of truncated replies to DNSSEC queries.

Heretofore, when a validating the result of an external query triggers
a DNSKEY or DS query and the result of that query is truncated, dnsmasq
has forced the whole validation process to move to TCP by returning a
truncated reply to the original requestor. This forces the original
requestor to retry the query in TCP mode, and the DNSSEC subqueries
also get made via TCP and everything works.

Note that in general the actual answer being validated is not large
enough to trigger truncation, and there's no reason not to return that
answer via UDP if we can validate it successfully. It follows that
a substandard client which can't do TCP queries will still work if the
answer could be returned via UDP, but fails if it gets an artifically
truncated answer and cannot move to TCP.

This patch teaches dnsmasq to move to TCP for DNSSEC queries when
validating UDP answers. That makes the substandard clients mentioned
above work, and saves a round trip even for clients that can do TCP.

src/domain-match.c

@@ -744,6 +744,7 @@ int add_update_server(int flags,
   serv->flags = flags;
   serv->domain = alloc_domain;
   serv->domain_len = strlen(alloc_domain);
+  serv->tcpfd = -1;
   
   return 1;
 }