In summary, wsrep_node_address and wsrep_sst_receive_address can now
be set to IPv6 addresses escaped by []. Rsync SST works out ouf the
box thanks to rsync daemon listening on both IPv4 and IPv6 sockets by
default. For xtrabackup SST onver IPv6 one needs to set sockopt in
the [sst] section of joiner's configuration file to ",pf=ip6" if
using socat as a streamer or to "-6" if using netcat.