[Enhancement] [RHEL/6] Add remediations for two IPv6 rules

This proposal adds (RHEL-6) remediations for following two rules:
* kernel_module_ipv6_option_disabled &
* network_ipv6_disable_rpc

Note: In the second remediation script we prefer to delete corresponding ```tcp6``` & ```udp6```  entries from ```/etc/netconfig``` (rather than to comment them out) to prevent their (un)intentional future re-application.

Testing report:
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The proposed change has been tested on RHEL-6 on both archs (32-bit & 64-bit) & works as expected at both of them.

Please review.

Thank you, Jan.