[Enhancement] [RHEL/6] Enable GUI Warning Banner:
* change OVAL ID from 'banner_gui_enabled' to 'gconf_gnome_gdm_login_banner_enabled'
  to follow the (more widely used) 'gconf_gnome_...' form present in other GConf2 rules,
* couple of OVAL check cosmetic changes (unify indentation, update test_attestation etc.),
* change rule ID in XCCDF & profile references to use the OVAL one,
* add remediation