Confirm MySQL SET forms that change server state

SET PASSWORD, SET GLOBAL and SET PERSIST (also as @@GLOBAL.x) change
the server or the database account rather than the session, so they
need confirmation like other writes. Session variables stay a read.

tests/Unit/Service/SqlGuardTest.php

@@ -71,6 +71,8 @@ class SqlGuardTest extends TestCase
             ['SHOW search_path'],
             ['EXPLAIN SELECT 1'],
             ["SET myvar.uid = 'alice'"],
+            ['SET @uid = 5'],
+            ['SET SESSION sql_mode = \'\''],
             ["SELECT \$\$DELETE FROM x\$\$"],
             ["SELECT E'it\\'s delete' FROM t"],
             ['VALUES (1)'],
@@ -109,6 +111,10 @@ class SqlGuardTest extends TestCase
             ['EXPLAIN ANALYZE DELETE FROM t'],
             ['SET ROLE postgres'],
             ['SET SESSION AUTHORIZATION postgres'],
+            ["SET PASSWORD = 'x'"],
+            ['SET GLOBAL max_connections = 10'],
+            ['SET PERSIST max_connections = 10'],
+            ['SET @@GLOBAL.max_connections = 10'],
             ['SELECT pg_terminate_backend(123)'],
             ["SELECT set_config('x', 'y', false)"],
             ["SELECT setval('seq', 1)"],