Make util/find-doc-nits runnable from the build tree

Make util/find-doc-nits runnable from the build tree

Because we generate an increasing number of POD files, some of them
end up in the build tree.  This makes it difficult for find-doc-nits
to work as desired when the build tree is separate from the source
tree.

The best supported way to make it work in such an environment is to
run it from the build tree and let it use the build information from
configdata.pm to find all the POD files.  To make this smooth enough,
we add a function 'files' that returns an array of file names
corresponding to criteria from the caller.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11045)