This function can be used for automatic protocol upgrade described in
[1].

Both unit tests and runnable example are included. Only the latter
requires systemd, as unit tests are self-sufficient, and only test
that JOURNAL_STREAM environment variable is checked properly.

[1] https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading

See discussion in coreos#410

A test that will be added in a latter commit requires systemd-run --wait,
which is not available on Ubuntu 16.04.

DEBIAN_FRONTEND=noninteractive prevents hang on "Setting up tzdata"
that became a problem on Ubuntu 20.04 (though something similar
could happen on any Debian/Ubuntu version).

To ensure that StderrIsJournalStream properly works in real conditions,
this test re-executes itself with systemd-run, and observes exit code
and logged entries.