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().

tests/Unit/Service/SqlGuardTest.php

@@ -110,6 +110,7 @@ class SqlGuardTest extends TestCase
             ['SELECT pg_terminate_backend(123)'],
             ["SELECT set_config('x', 'y', false)"],
             ["SELECT setval('seq', 1)"],
+            ["SELECT nextval('seq')"],
             ['MERGE INTO t USING s ON true WHEN MATCHED THEN DELETE'],
             ['VACUUM'],
             ["WITH a AS (SELECT '/*'), b AS (DELETE FROM t RETURNING 1) SELECT '*/'"],