Allow passing directories as the config sources

We should allow passing the entire directories as the config sources. In
this case CouchDB will traverse the given directories and load every
*.ini file found.

E.g.

"... -couch_ini /etc/couchdb/default.d/ /etc/couchdb/local.d/ /path/to/extra/couchdb/default.ini /path/to/extra/couchdb/local.ini ..."

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
As noted in IRC, the ordering of filelib:wildcard is not specified, so this is unpredictable.

To fix this, let's have [V | AccIn] for the earlier clause, preserve this clause, but add a final lists:sort(Values) for the function's result.