These fixes allow *nix environments to connect to SQL Server using
the dblib PDO driver and the silverstripe mssql module.
- Only set MYSQL_ATTR_INIT_COMMAND when using the mysql driver, this
constant isn't defined if the mysql pdo driver isn't installed
- Supress warnings on getting the server version, attributes aren't
supported by the dblib driver
- Explicitly check for errors on sql exec, checking the return
value isn't reliable for statements with no return value (e.g.
USE database)
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