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.