Move to PDO for all database access

This can't be done until we're on D7 with the new db layer.

so this is a very very long time away.

We now no longer rely on the drupal database layer at all, and drush 2.x now requires php 5.2, so we can rely on PDO always being available.

The only functions that need to differ for PG are :

function _provision_mysql_new_site_db($db_name, $db_user, $db_passwd, $db_host) {}
function _provision_mysql_destroy_site_db($db_name, $db_user, $db_passwd, $db_host) {}
function _provision_mysql_database_exists($name) {}
function _provision_mysql_create_database($name) {}
function _provision_mysql_grant($name, $username, $password, $host = '') {}
function _provision_mysql_revoke($name, $username, $host = '') {}
function _provision_mysql_import_dump($dump_file, $db_name, $db_user, $db_passwd, $db_host) {}

At least for D6 sites, this should allow us to have full postgres support for drupal (in as far as the drupal postgres port actually works).

This would also give us access to sqlite, which might be useful for some interesting problems.

we need to move them into drush engines, and conditionally include them based on a flag.

i'm open to making them classes too.

in fact, this might help a lot in our future work for server stuff.

this is done in dev-services.