Allow preference of IPv6 for source_addr in websockify

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

utils/websocket.py

@@ -109,6 +109,7 @@ 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
@@ -801,7 +802,7 @@ 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)

utils/websockify

@@ -242,6 +242,9 @@ 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