Convince macOS to do ipv6 dns lookups

Puts a value into System Config, similar to how DNS push works.

closes #1466

https://apple.stackexchange.com/questions/309430/ipv6-dns-resolution-on-macos-high-sierra

osdep/MacDNSHelper.hpp

@@ -3,6 +3,7 @@
 
 #include <vector>
 #include "../node/InetAddress.hpp"
+#include "../node/MAC.hpp"
 
 namespace ZeroTier {
 
@@ -11,6 +12,8 @@ class MacDNSHelper
 public:
     static void setDNS(uint64_t nwid, const char *domain, const std::vector<InetAddress> &servers);
     static void removeDNS(uint64_t nwid);
+    static bool addIps(uint64_t nwid, const MAC mac, const char *dev, const std::vector<InetAddress> &addrs);
+    static bool removeIps(uint64_t nwid);
 };
 
 }