lsm: add support for c/ring LSM profiles

This patch adds support for checkpoint and restore of two linux security
modules (apparmor and selinux). The actual checkpoint or restore code isn't
that interesting, other than that we have to do the LSM restore in the restorer
blob since it may block any number of things that we want to do as part of the
restore process.

I tried originally to get this to work using libraries in the restorer blob,
but I could _not_ get things to work correctly (I assume I was doing something
wrong with all the static linking, you can see my draft attempts here:
https://github.com/tych0/criu/commits/apparmor-using-libraries

 ). I can try to
resurrect this if it makes more sense, to do it that way, though.

v2: lsm_profile lives in creds.proto instead of the task core, look in a more
    canonical place for selinuxfs and don't try to special case any selinux
    profile names.
v3: only allow unconfined selinux profiles

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>