[IPV6] MROUTE: Support PIM-SM (SSM).

Based on ancient patch by Mickael Hoerdt
<hoerdt@clarinet.u-strasbg.fr>, which is available at
<http://www-r2.u-strasbg.fr/~hoerdt/dev/linux_ipv6_mforwarding/patch-linux-ipv6-mforwarding-0.1a

>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>