journald-mock: do not use pipe2() which is linux(glibc>=2.9)-specific

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>
systemd-journal: use the sd_journal_get_realtime_usec instead of _SOURCE_REALTIME_TIMESTAMP field

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
systemd-journal: Added debug message for incoming messages from journal.

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
systemd-journal: only one instance is acceptable

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
systemd-journal: if _COMM field does not exists, use the PROGRAM_IDENTIFIER field.

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>
systemd-journal: add seek_to_tail function to journald_subsystem

Signed-off-by: Juhász Viktor <viktor.juhasz@balabit.com>