Add abiltiy to search systemd journals by syslog identifiers. Debian …

[hpreusse]

See Debian Bug#1060859.

I scan mail logs for Deliverable Status Notifications for 'dsn=5.7" on my delivery server, so if some recipient starts blocking the server I can start investigating and switch to a different outbound server before users start to report that 'mail is not working'.

For systems where mail logs are in /var/log/mail.log this is simple, but where they are in the systemd journal a scan which looks only at, in this case, entries with a SYSLOG_IDENTIFIER of 'postfix/smtp' is more effective.

The attached patch allows an argument of --type=journald:identifier='postfix/smtp' to be specified.