Add SMTPS/STARTLS support

Open username-removed-1178112 requested to merge J08nY/mailman:mta-smtps-starttls into master

This MR adds SMTPS and STARTTLS support. Fixes #252.

The tests use a self-signed certificate for localhost with these fields:

    Subject: C = US, ST = MA, O = Mailman test certificate, CN = localhost

I used the values for FSF, since Mailman is a GNU project and nothing better came to mind.

Edited by username-removed-1178112