Add support for postfix/submission/smtpd matching.

By default, on debian, and perhaps globally, the postfix smtpd daemon running on port 587 logs with the daemon name

postfix/submission/smtpd

instead of

postfix/smtpd

This covers both cases (old, new, port 25, port 587).

Close #803
it indeed does look like a possible attack vector but per se might also happen in non-malicious cases.  So is not a clear indicator... may be worth for smth like postfix-dos  filter but IMHO not for this one.
similar argumentation for the one below -- couldn't it happen simply on unstable connection without any malicious attempt?

so let's keep this PR solely for the original purpose.