Some platforms need two sockets with AF_INET and AF_INET6 to listen to both
protocols.
This patch changes the single listen socket each vhost could previously
handle to become an lws_dll2 and adapts the related code to handle them as
a linked-list rather than as a singleton.
The next patch adapts the listen / server code to create multiple listen
wsi for vhosts listening on multiple ip protocols.
