Porter should automatically select best installed SQL extension

[linc]

The current stable release of Porter (2.4, Oct 2018) has hardcoded mysql_* functions in it, creating problems for folks now missing this extension (it is long since deprecated).

Examples:

• https://open.vanillaforums.com/discussion/36921/vanilla-porter-version-2-4-with-php-7-x
• https://open.vanillaforums.com/discussion/37468/migrating-from-phpbb

In the latest master there appears to have been a config.php added that defines a DB_EXTENSION to be used, one of mysql, mysqli, or pdo. The default is PDO. The issue with this is that we're adding a configuration step that is nearly impossible with how make.php works; it creates a giant monolithic file (by design) that will be challenging for the user to edit. (I should note at this point I'm not even sure that make.php has been updated to work with the new DB setup.)

Rather than manually setting the DB_EXTENSION, it should automatically chosen based on extension availability. Prefer PDO, then mysqli, then mysql. That absolves the user of needing to select, and the preference order is fairly clear so I don't see a drawback to doing this automatically.

Let's please get a working version of this released to open source with priority now that we know the current version is going to fail for modern servers.