audit: update shutdown LSM data

The audit process LSM information is changed from a secid audit_sig_sid
to an lsm_prop in audit_sig_lsm. Update the users of this data
appropriately. Calls to security_secid_to_secctx() are changed to use
security_lsmprop_to_secctx() instead. security_current_getsecid_subj()
is scaffolded. It will be updated in a subsequent patch.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
[PM: subject line tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>