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

include/curl/curl.h

@@ -1844,6 +1844,9 @@ typedef enum {
   /* send HAProxy PROXY protocol header? */
   CINIT(HAPROXYPROTOCOL, LONG, 274),
 
+  /* shuffle addresses before use when DNS returns multiple */
+  CINIT(DNS_SHUFFLE_ADDRESSES, LONG, 275),
+
   CURLOPT_LASTENTRY /* the last unused */
 } CURLoption;