The session id is the pid of the leader process of the current session.
It may be the shell handling the autocomplete in foreground but not necessary.

A leader shell may spawn another shell and it is this shell who owns the
terminal and it is handling the autocomplete. Sending a SIGWINCH to the
former shell makes no effect.

The only reliable way to know the pid of the correct shell is
retrieving it from the autocomplete script and passing it to byexample
via an environment variable.

The user is expected to do something like

```
eval $(byexample -x-autocomplete bash)
```

or

```
byexample -x-autocomplete bash > src
source ./src
```

With this, the shell should be configured and autocompletion for
byexample enabled.

Currently only bash is supported.

Instead of using the source code directly, build a package and install
byexample from it and only then run the tests.
This is meant to catch some building issues like missing data files.