tproxy: use the interface primary IP address as a default value for --on-ip

The REDIRECT target and the older TProxy versions used the primary address
of the incoming interface as the default value of the --on-ip parameter.
This was unintentionally changed during the initial TProxy submission and
caused confusion among users.

Since IPv6 has no notion of primary address, we just select the first address
on the list: this way the socket lookup finds wildcard bound sockets
properly and we cannot really do better without the user telling us the
IPv6 address of the proxy.

This is implemented for both IPv4 and IPv6.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: KOVACS Krisztian <hidden@balabit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>