This adds the lxc.seccomp.allow_nesting api extension. If
lxc.seccomp.allow_nesting is set to 1 then seccomp profiles will be
stacked. This way nested containers can load their own seccomp policy on
top of the policy that the outer container might have applied.

Cc: Simon Fels <simon.fels@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>