pid namespace and namespace cleanups

Hi,

The next serie contains some cleanups on namespaces that have been
lying in our container patchset for a while. The patches seem stable
enough and not too controversial to be good candidates for a -mm merge.

Here's a short summary of what they are doing :

* add a process_session() helper routine
* rename 'struct namespace' to 'struct mnt_namespace' 
* add pid namespace framework to nsproxy but without the unshare
  feature
* add a child_reaper per pid namespace
* add an id to nsproxy for a future syscall bind_ns

A next serie should follow soon, which depends on the above. 

* net namespace framework, empty object without any dependency on
  the layer
* unshare_ns syscall (unshare dedicated to namespaces with its own 
  flags) 
* bind_ns syscall enabling a process to switch nsproxy
* more cleanups for the pid namespace

Pending patches we are still working on :

* layer 3 net namespace providing a simple and fast isolation of
  a net interface
* full pid namespace, this is quite complex.

Thanks,

C.