Re: [OpenSCAP/scap-security-guide] SSG updates for NIST 800-53 rev4? (#2135)

References attached. Input was RHEL7 XCCDF build from master branch. refs.zip <https://github.com/OpenSCAP/scap-security-guide/files/1155357/refs.zip> The origins of these are found subordinate to |shared/xccdf| (there are 853 occurrences, many of which have no NIST citation, with 5 others in |RHEL/7/input/auxiliary/nist_support.xml|). The "raw" citations reference only controls, not revision+control, so any cleanup accommodating multiple 800-53 revisions would require altering not only the |<ref>|s but the build logic.