MINOR: cfgparse: add cfg_find_best_match() to suggest an existing word

MINOR: cfgparse: add cfg_find_best_match() to suggest an existing word

Instead of just reporting "unknown keyword", let's provide a function which
will look through a list of registered keywords for a similar-looking word
to the one that wasn't matched. This will help callers suggest correct
spelling. Also, given that a large part of the config parser still relies
on a long chain of strcmp(), we'll need to be able to pass extra candidates.
Thus the function supports an optional extra list for this purpose.