mroute was using /proc/net/ip_mr_[vif|cache] to display mroute entries. Hence,
only RT_TABLE_DEFAULT was displayed and only IPv4.
With rtnetlink, it is possible to display all tables for IPv4 and IPv6. The output
format is kept. Also, like before the patch, statistics are displayed when user specify
the '-s' argument.

The patch also adds the support of 'ip monitor mroute', which is now possible.
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>