[Enhancement] Port single_user_auth to RHEL7 and Fedora

- Update RHEL7 and Fedora XCCDF for require_singleuser_auth
- Update require_singleuser_auth OVAL for RHEL7 and Fedora
Fix typos in OVAL
Update OCIL clause for single-user-auth

- Update OCIL clause for RHEL7 and Fedora
Merge pull request #261 from redhatrises/single_user_auth

[Enhancement] Port single_user_auth to RHEL7 and Fedora