sit: stateless autoconf for isatap

be sent periodically. The rs_delay can be speficied when adding the
PRL entry and defaults to 15 minutes.

The RS is sent from every link local adress that's assigned to the
tunnel interface. It's directed to the (guessed) linklocal address
of the router and is sent through the tunnel.

Better: send to ff02::2 encapsuled in unicast directed to router-v4.

Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Signed-off-by: David S. Miller <davem@davemloft.net>