support get_config_item(lxc.seccomp) and clear_config_item(lxc.seccomp)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
attach: try to use the container's seccomp policy

We can't get the actual policy (in the case where the policy file
has changed) from the container, but at least we can use the
seccomp policy file listed in the container config file.

(If anyone wants to further improve this, it may be better to get
the seccomp policy over the cmd api;  not sure that's what we want,
and this seems simpler to hook into the existing code, so I went
this way for now)

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>