[php-src] Issue #7818: Add mysqli_rollback_to

Issue: https://github.com/php/php-src/issues/7818 Author: KapitanOczywisty ### Description In PHP 5.5 mysqli::savepoint and mysqli::release_savepoint were added, but not rollback_to. Maybe there is some database engine where savepoints have multiple purposes, but in mariadb the only use of savepoints is to call query ROLLBACK TO name query manually. For completion: existing rollback with $name is doing nothing (only adding comment) at least in mariadb/mysql. Method signature ```php class mysqli { /* ... */ public function rollback_to(string $name) : bool; } ``` Sample test sequence ```php mysqli_query($link, 'DROP TABLE IF EXISTS test'); mysqli_query($link, 'CREATE TABLE test(id INT) ENGINE = InnoDB'); mysqli_begin_transaction($link); mysqli_query($link, 'INSERT INTO test(id) VALUES (1)'); mysqli_savepoint($link, 'foo'); mysqli_query($link, 'INSERT INTO test(id) VALUES (2)'); mysqli_rollback_to($link, 'foo'); mysqli_commit($link); $res = mysqli_query($link, "SELECT * FROM test"); var_dump($res->num_rows); // int(1) ```