[BugFix] [RHEL/6] Fix sysctl_net_ipv6_conf_default_accept_ra OVAL check's false positive -
when IPv6 support is disabled, don't perform the runtime sysctl configuration check (since
in that case the required flag won't be set => the check will fail even when it should pass).

This aligns the implementation with OVAL check for other IPv6 net
sysctl_net_ipv6_conf_default_accept_redirects option which performs the scan correctly.
[RHEL/6] audit_config_immutable rule:
* Unify XCCDF ID with OVAL ID (in XCCDF & also in profile references
so the remediation call would work),
* Add RHEL-6 remediation.
Merge pull request #356 from iankko/rhel6_audit_immutable

[RHEL/6] audit_config_immutable rule - Unify XCCDF ID with OVAL ID & add remediation
Merge pull request #355 from iankko/rhel6_ipv6_accept_ra

[BugFix] [RHEL/6] Fix sysctl_net_ipv6_conf_default_accept_ra false positive