systemd-journal: Prefer SYSLOG_IDENTIFIER over _COMM

In order to not break assumptions, prefer `SYSLOG_IDENTIFIER` over `_COMM`. For example, postfix uses `postfix/qmgr` as `SYSLOG_IDENTIFIER`, but `_COMM` is only `qmgr`. The journal itself uses `SYSLOG_IDENTIFIER` when reconstructing the syslog message, so we should not deviate from that behaviour, either.

Similarly, rsyslog also prefers `SYSLOG_IDENTIFIER`, so for the sake of compatibility, doing the same is preferable.

Reported-by: Alexander Görtz
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>