samples: add an example of seccomp user trap

The idea here is just to give a demonstration of how one could safely use
the SECCOMP_RET_USER_NOTIF feature to do mount policies. This particular
policy is (as noted in the comment) not very interesting, but it serves to
illustrate how one might apply a policy dodging the various TOCTOU issues.

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Kees Cook <keescook@chromium.org>