UBUNTU: SAUCE: LSM stacking: LSM: general but not extreme module stacking

UBUNTU: SAUCE: LSM stacking: LSM: general but not extreme module stacking

Leverage the infrastructure management of the credential and
file security blobs to allow stacking of security modules in
all but the most extreme case. Security modules are informed
of the location of their data within the blobs at module
initialization.

Stacking is optional. If stacking is not configured the old
limit of one "major" security module applies. If stacking is
configured any combination that does not include both SELinux
and Smack is allowed.

A subdirectory has been added to /proc/.../attr for each of
SELinux and AppArmor (Smack introduced such a subdirectory earlier)
to disambiguate what data is provided in the proc/.../attr
interfaces. An entry "context" is added to /proc/.../attr and
to each of the subdirectories. The "context" entry provides
process attribute information in the form:

        lsm-name='lsm-data'[,lsm-name='lsm-data']...

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>