PDO: Add support for SQLite open flags

[bohwaz]

This allows to open a SQLite database in read-only, just like with the SQLite3 extension.

Example usage:

$pdo = new PDO('/tmp/db.sqlite', null, null, [
    PDO::SQLITE_ATTR_OPEN_FLAGS => PDO::SQLITE_READONLY
]);

[krakjoe]

Merged ed2f651

Thanks.