resolve: add CURLOPT_DNS_SHUFFLE_ADDRESSES

This patch adds CURLOPT_DNS_SHUFFLE_ADDRESSES to explicitly request
shuffling of IP addresses returned for a hostname when there is more
than one. This is useful when the application knows that a round robin
approach is appropriate and is willing to accept the consequences of
potentially discarding some preference order returned by the system's
implementation.

Closes #1694

tests/data/Makefile.inc

@@ -177,6 +177,7 @@ test1533 test1534 test1535 test1536 test1537 test1538 \
 test1540 \
 test1550 test1551 test1552 test1553 test1554 test1555 test1556 \
 test1600 test1601 test1602 test1603 test1604 test1605 test1606 test1607 \
+test1608 \
 \
 test1700 test1701 test1702 \
 \

tests/data/test1608

@@ -0,0 +1,26 @@
+<testcase>
+<info>
+<keywords>
+unittest
+curlopt_dns_shuffle_addresses
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+</features>
+ <name>
+verify DNS shuffling
+ </name>
+<tool>
+unit1608
+</tool>
+</client>
+
+</testcase>