newer versions of Python do not allow binding of sockets 'by hand'
anymore. Interestingly, everything works perfectly without. Was
it ever needed? Not sure.

now terminals can be initialized with either absolute or relative only,
the other one being chosen sensibly. Also remove race condition when
searching for free addresses.

this way we can read individual object entries without reading
the entire list.

and also improve its output a bit

on some terminals, the PDO can be configured. This
allows for that. It is not perfect yet as we do not properly
overwrite existing PDOs.

uses some internal marked stuff, forgot about the details

now we can keep many terminals operational within the same
EtherCAT roundtrip

until now we only had one asynchronous packet on the wire.
This allows to send them as fast as possible, waiting for them
in bulk.