Start implementing support for ipv6.

Start implementing support for ipv6.

This enhances the framework for sending tcp tickles to be able to send ipv6 tickles as well.

Since we can not use one single RAW socket to send both handcrafted ipv4 and ipv6 packets, instead of always opening TWO sockets, one ipv4 and one ipv6 we get rid of the helper ctdb_sys_open_sending_socket() and just open (and close)  a raw socket of the appropriate type inside ctdb_sys_send_tcp().
We know which type of socket v4/v6 to use based on the sin_family of the destination address.

Since ctdb_sys_send_tcp() opens its own socket  we no longer nede to pass a socket
descriptor as a parameter.  Get rid of this redundant parameter and fixup all callers.