[RHEL/6] audit_config_immutable rule - Unify XCCDF ID with OVAL ID & add remediation

* Unify XCCDF ID with OVAL ID (in XCCDF & also in profile references
so the remediation call would work),
* Add RHEL-6 remediation

Tested on both {32-,64-}bit RHEL-6 versions & works as expected.

Please review.

Thank you, Jan.