[Mail] Merge MTA and MDA, add doc, build via Compose and secrets

pica-mail/pica-mail-mta/spam/nov2018.picasoft.net.rsa

pica-mail/pica-mail-mta/spam/opendkim

pica-mail/pica-mail-mta/spam/opendkim.conf

pica-mail/pica-mail-mta/spam/opendmarc

pica-mail/pica-mail-mta/spam/opendmarc.conf

pica-mail/pica-mail-mta/test-auth_local_user/Dockerfile

pica-mail/pica-mail-mta/test-auth_local_user/README.md

pica-mail/pica-mail-mta/test-auth_local_user/entrypoint2.sh

pica-mail/pica-mail-mta/test-auth_local_user/local_users

pica-mail/secrets/mail.secrets.example