Introduce lsm hooks for kdbus

This is combination of work by Karol Lewandowski and Paul Moore
on LSM hooks for kdbus.

[v1 Initial version]
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>

[v2 added: memfd_seal
    added: bus_alloc/free
    added: ep_create ; ep_setpolicy ; ep_create
    added: read ; write ]
Signed-off-by: Paul Moore <pmoore@redhat.com>

[v3 Rebased on top of 11f6693c1 (compatible with systemd v212),
    dropped: memfd_seal - will be addressed in separately,
    renamed: ep_create -> ep_alloc (for consistency),
    renamed: send+recv -> talk,
    renamed: read -> recv ; write -> send,
    added: domain_alloc/free
    added: ep_free ]
Signed-off-by: Karol Lewandowski <k.lewandowsk@samsung.com>