[php-src] Issue #7818: Add mysqli_rollback_to

Issue: https://github.com/php/php-src/issues/7818 Comment Author: KapitanOczywisty I have no idea why this is implemented as a comment, maybe for debugging or to be similar to other api or maybe named transactions were planed at the time. Also seems like ROLLBACK TO was available at that time, but there is distinction to mention: ROLLBACK and ROLLBACK TO are in fact different statements - the first one is reverting whole transaction, the second one is only reverting to the named savepoint. ROLLBACK have options [NO] CHAIN and [NO] RELEASE ($flag argument) which ROLLBACK TO does not. It's probably better to keep them as separate functions.