Add ressource deallocation

Disconnect and close all handles. This must be done in the correct
order or SQLFreeHandle will generate a "HY010 Function sequence error".

Refactor diagnostic to a central get_diagnostic_text () function.

UnixOdbcLL:

- Add SQLFreeHandle
- Add SQLDisconnect

UnixOdbc:

- Add free_handle_checked ()
- Add get_diagnostic_text ()
- Call free_handle_checked () and disconnect () where necessary
- Keep a reference to parent handle class
- Increase attribute buffer size to avoid truncation