postfixadmin php7-incompatible default configuration

[hanshenrik]

18.04's postfixadmin package defaults to $database_type="mysql"; in /etc/postfixadmin/dbconfig.inc.php , but 18.04 ships with PHP 7.2, and the "mysql" database driver doesn't work with PHP versions above 5.6, the 7.X compatible database driver is called "mysqli" - since 18.04 use PHP 7.2, the default should be the php7-compatible $database_type="mysqli"; , not the php7-incompatible "mysql"

(also about backwards-compatibility: the mysqli driver was introduced in php 5.0 released at July 2004, which is ancient at this point, but the much older mysql driver was introduced in PHP 2.0, released in November 1997)