incorporate log file timestamp into subject line when mailing log

[cgzones]

From https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=431601:

Often logs don't include year information, and once a log is emailed, 
the original file timestamps are lost.

I propose an enhancement that would grab the file timestamp and 
incorporate it into the subject line. Something like:

Subject: /var/log/syslog/mail.log [2006-06-02]


Under some circumstances, when using the "dateext" directive, the date 
extension will be present in the subject and meet this requirement, but 
it appears inconsistently (see my other bug report) and the above could 
work consistently regardless of whether "dateext" was in use.

Which file timestamp (mtime, ctime?) should be used for this purpose is 
another matter...

 -Tom

[kdudka]

I think this would need to be optional so that we do not break consumers of the current format of subject.