ipv6 per network namespace - intro

This patchset provides the changes to make the ipv6 protocol to handle multiple network namespaces.

There is a pre-requesite which is already merged in net-2.6.25, it is the separate workq for the network namespace cleanup. It will disappear when Denis will rebase the patchset to the latest net-2.6.25.

The sysctl per net is not yet implemented (coming soon), the patchset adds a flag to activate the network namespace only if sysctl is disabled. It will be reverted as soon as we handle sysctl.

This subset makes per namespace:

* inet6_addr
* ip6_fib
* fib6_rules
* rt6_stats
* rt6_info
* route6
* addrconf
* ndisc

The minimum set of functionalities we have with this patchset allows to setup the routes, ipv6 addresses per namespaces and check the proc files are isolated.