[Enhancement] [RHEL/6] RHEL-6 gconf rule's remediations round two

This change operates on the following three gconf's rules:
* ```Enable GUI Warning Banner```,
* ```Set GUI Warning Banner Text```, and
* ```Disable the User List```

(IOW those from RHEL/6/input/system/accounts/banners.xml) and applies the following changes to them:
* substitute former OVAL check name with 'gconf_gdm' prefix,
* update OVAL check if / where necessary (mostly indentation, test_attestation, new OVAL object names changes),
* change rule ID in XCCDF & in profile references (where appropriate) to start using the OVAL ID,
* and finally add remediation for that.

Basically it's 2-round of adding remediations for gconf rules (like in https://github.com/OpenSCAP/scap-security-guide/pull/334) but this time for gconftool-2 RHEL-6 rules from ```input/system/accounts/banners.xml``` file.

Testing report:
--
The change has been tested on RHEL-6 Server & works as expected (for all three rules).

Please review.

Thanks, Jan.