- When we detect that the container, we want to attach to, has been stared with
  PR_SET_NO_NEW_PRIVS we attach with PR_SET_NO_NEW_PRIVS as well. (We might
  relax this restriction later but let's be strict for now.)
- When LXC_ATTACH_NO_NEW_PRIVS is set in the flags passed to
  lxc_attach()/attach_child_main() then we set PR_SET_NO_NEW_PRIVS irrespective
  of whether the container was started with PR_SET_NO_NEW_PRIVS or not.
- Set no_new_privs before lsm and seccomp. We probably don't want attach() to
  be able to change the lsm or seccomp policy if the container was started with
  PR_SET_NO_NEW_PRIVS enabled.
Signed-off-by: Christian Brauner <christian.brauner@canonical.com>