[php-src] Issue #20394: Allow mysqli to connect through socket for any host

Issue: https://github.com/php/php-src/issues/20394 Author: JoDuENSCM ### Description Currently, mysqli can connect through socket only when the host is localhost, but in our security contexts we can only access our MySQL/MariaDB databases accessing distant socket through SSH tunnel. Is it possible to change the behavior so that when a socket is specified (or a non empty string), the connection goes through specified socket? Currently, the MySQL extensions of PDO allows such connections, we cannot migrate from PDO to mysqli (plus several projects such as phpMyAdmin only make use of mysqli). Regards