Refine cchech.sh and config file to allow specification of 'Unset'
to be required, in which case we pass the check if the value is
unset regardless of the default value.
We already allow checking the default value and consider it to comply
if the default value matches expected. This new change will support
some values being defined as required to be unset.
Add examples and descriptive text in configurables.cfg.
Fix ShellCheck compliance issues.
Changed '-v' so required values having expected values show as GREAT
rather than GOOD. Recommended values having expected values still
show as GOOD. For values *not* matching expectations, required values
show as BAD, while recommended values now show as WARN.
The word FAIL is now used only in the summary, and appears only if
there are any BAD indications (required values not matching) or else
any system errors checking configurables.