`shell_interact()` is currently not nice to use. If you try to cancel
the socat process, it will also break the nixos test. Furthermore
ptpython creates it's own terminal that subprocesses are running in,
which breaks some of the terminal features of socat.
Hence this commit extends `shell_interact` to allow also to connect to
arbitrary servers i.e. tcp servers started by socat.
Previously the driver was configured exclusively through convoluted
environment variables.
Now the driver's defaults are configured through env variables.
Some additional concerns are in the github comments of this PR.
