Merge PDO_*_Ext definitions into PDO class

[ondrejmirtes]

In php-src there are some special stubs that are not supposed to be used on their own because they don't represent any real class, like: https://github.com/php/php-src/blob/master/ext/pdo_pgsql/pgsql_driver.stub.php

/cc @kocsismate Are you aware of any other special stubs that need special handling in php-src? Thanks!

[kocsismate]

Yes, there is PDO_SQLite_Ext. But I'm not aware of any other special stub.

[kocsismate]

I mean, there is a few weirdness which you may already be aware of:

• SAPIs alias the same function many times (e.g. getallheaders())
• there are a few test extensions: zend_test, dl_test and skeleton