Allow preference of IPv6 for source_addr

Add the option "-6, --prefer-ipv6". When set, the 'prefer_ipv6' flag in
websocket.py is used so that 'source_addr' is resolved to IPv6 if
available. If 'source_addr' is not set, binds to [::].

websocket.py

@@ -110,6 +110,7 @@ Sec-WebSocket-Accept: %s\r
         self.verbose        = verbose
         self.listen_host    = listen_host
         self.listen_port    = listen_port
+        self.prefer_ipv6    = source_is_ipv6
         self.ssl_only       = ssl_only
         self.daemon         = daemon
         self.run_once       = run_once
@@ -814,7 +815,7 @@ Sec-WebSocket-Accept: %s\r
         is a WebSockets client then call new_client() method (which must
         be overridden) for each new client connection.
         """
-        lsock = self.socket(self.listen_host, self.listen_port)
+        lsock = self.socket(self.listen_host, self.listen_port, False, self.prefer_ipv6)
 
         if self.daemon:
             self.daemonize(keepfd=lsock.fileno(), chdir=self.web)

websockify

@@ -266,6 +266,9 @@ def websockify_init():
             choices=["exit", "ignore", "respawn"],
             help="action to take when the wrapped program exits "
             "or daemonizes: exit (default), ignore, respawn")
+    parser.add_option("--prefer-ipv6", "-6",
+            action="store_true", dest="source_is_ipv6",
+            help="prefer IPv6 when resolving source_addr")
     (opts, args) = parser.parse_args()
 
     # Sanity checks