Introduce support for seccomp.

Introduce support for seccomp.

Hi,

This patch is so far just a proof of concept.  The libseccomp api will be
changing soon so it probably wouldn't be worth pulling this until it is
updated for the new API.

This patch introduces support for seccomp to lxc.  Seccomp lets a program
restrict its own (and its children's) future access to system calls.  It
uses a simple whitelist system call policy file.  It would probably be
better to switch to something more symbolic (i.e specifying 'open' rather
than the syscall #, especially given container arch flexibility).

I just wanted to get this out there as a first step.  You can also get
source for an ubuntu package based on this patch at
https://code.launchpad.net/~serge-hallyn/ubuntu/quantal/lxc/lxc-seccomp

Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>