OpenVZ kernel based checkpointing/restart

This patchset introduces kernel based checkpointing/restart as it is
implemented in OpenVZ project. This patchset has limited functionality and
are able to checkpoint/restart only single process. Recently Oren Laaden
sent another kernel based implementation of checkpoint/restart. The main
differences between this patchset and Oren's patchset are:

* In this patchset checkpointing initiated not from the process
(right now we do not have a container, only namespaces), Oren's patchset
performs checkpointing from the process context.

* Restart in this patchset is initiated from process, which restarts a new
process (in new namespaces) with saved state. Oren's patchset uses the same
process from which restart was initiated and restore saved state over it.

* Checkpoint/restart functionality in this patchset is implemented as a kernel
module


As checkpointing is initiated not from the process which state should be saved
we should freeze a process before saving its state. Right now Container Freezer
from Matt Helsley can be used for this.

This patchset introduce only a concept how kernel based checkpointing/restart
can be implemented and are able to checkpoint/restart only a single process
with simple VMAs. 

I've tried to split my patchset in small patches to make review more easier.