Pass the -lock file back via _usednames. This seems to be reliable

if using MDB_NOSUBDIR per http://www.lmdb.tech/doc/group__mdb.html#ga32a193c6bf4d7d5c5d579e71f22e9340

dbm/apr_dbm_lmdb.c

@@ -343,7 +343,7 @@ static void vt_lmdb_usednames(apr_pool_t *pool, const char *pathname,
                               const char **used1, const char **used2)
 {
     *used1 = apr_pstrdup(pool, pathname);
-    *used2 = NULL;
+    *used2 = apr_pstrcat(pool, pathname, "-lock", NULL);
 }