- Use LMDB instead of BDB in preparation of obsoleting BDB

- Add "SMTP_TLS_SECURITY_LEVEL" confguration variable

- Add "NULLCLIENT" support for pure relayhosts

- Use mailLocalAddress (LDAP) for mail aliases, too.

- Rework container description
- Fix virtual domain handling: don't use environment variable and
LDAP at the same time.

- Add support for spamassassin

- Set local_header_rewrite_clients to static:all if domain
masquerading is requested (required for postfix >= 2.2)

- Fix error caused by uninitialized DEBUG variable