Reapply "Allow binding to IPv6 (if present)"

This reverts commit 61712d322a.

RNS/Interfaces/TCPInterface.py

@@ -412,6 +412,11 @@ class TCPServerInterface(Interface):
     def get_address_for_if(name):
         import RNS.vendor.ifaddr.niwrapper as netinfo
         ifaddr = netinfo.ifaddresses(name)
+
+        # IPv6 preference (if present)
+        if(netinfo.AF_INET6 in ifaddr):
+            return ifaddr[netinfo.AF_INET6][0]["addr"]
+        
         return ifaddr[netinfo.AF_INET][0]["addr"]
 
     @staticmethod