Support IPv6 lookup in bitcoin-cli even when IPv6 only bound on localhost

First query in the current way (intelligently determining which network
has a non-localhost interface). If this does not succeed, try plain
lookup.

Needed for testing.

Fixes #1827 by always allowing IPv6 to be used.

src/rpcprotocol.h

@@ -103,11 +103,27 @@ public:
     }
     bool connect(const std::string& server, const std::string& port)
     {
-        boost::asio::ip::tcp::resolver resolver(stream.get_io_service());
-        boost::asio::ip::tcp::resolver::query query(server.c_str(), port.c_str());
-        boost::asio::ip::tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);
-        boost::asio::ip::tcp::resolver::iterator end;
+        using namespace boost::asio::ip;
+        tcp::resolver resolver(stream.get_io_service());
+        tcp::resolver::iterator endpoint_iterator;
+#if BOOST_VERSION >= 104300
+        try {
+#endif
+            // The default query (flags address_configured) tries IPv6 if
+            // non-localhost IPv6 configured, and IPv4 if non-localhost IPv4
+            // configured.
+            tcp::resolver::query query(server.c_str(), port.c_str());
+            endpoint_iterator = resolver.resolve(query);
+#if BOOST_VERSION >= 104300
+        } catch(boost::system::system_error &e)
+        {
+            // If we at first don't succeed, try blanket lookup (IPv4+IPv6 independent of configured interfaces)
+            tcp::resolver::query query(server.c_str(), port.c_str(), resolver_query_base::flags());
+            endpoint_iterator = resolver.resolve(query);
+        }
+#endif
         boost::system::error_code error = boost::asio::error::host_not_found;
+        tcp::resolver::iterator end;
         while (error && endpoint_iterator != end)
         {
             stream.lowest_layer().close();