Show rows for all reads and confirm nextval()

SHOW, EXPLAIN, VALUES and TABLE count as reads now but their rows were
dropped and reported as a write; every read except SET now returns its
rows. nextval() advances a sequence, so it needs confirmation like
setval().

lib/Controller/DbController.php

@@ -174,7 +174,7 @@ class DbController extends Controller
                 $stmt = $this->db->prepare($query);
                 $stmt->execute();
 
-                if ($statementType === 'SELECT' || ($statementType === 'WITH' && SqlGuard::isReadOnly($query))) {
+                if ($statementType === 'SELECT' || ($statementType !== 'SET' && SqlGuard::isReadOnly($query))) {
                     // Read row by row and stop at MAX_ROWS, instead of
                     // fetchAll() + array_slice — otherwise a SELECT without
                     // LIMIT on a huge table would still pull everything into