journald-source: New source module is implemented

The module does not require installed systemd-journal development package
Unit tests also implemented and these tests can run on non-journald systems

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
systemd-journal: make it possible to turn on/off systemd-journal usage

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
Merge pull request #229 from juhaszviktor/3.6/f/journald-source

journald-source: New source module is implemented