TLS_TODO

This list does not really follow priority.

* Code cleanup: split smtp_session.c into generic SMTP, legacy TLS,
  and current TLS.  The amount of TLS code now dominates the file.
  Do this after all other code revisions stabilize, to avoid
  complicating code reviews.

* Code cleanup: TLS_LEV_NOTFOUND no longer belongs in the TLS
  library. It is an SMTP-client only feature. To fix, change the
  policy lookup API and use a different method to indicate if a
  policy was found. At the same time, fix policy lookup to initialize
  session->tls_level.

* Code cleanup: see if multiple consecutive switches can be aggregated
  (set_cipher_grade() and session_tls_init()).

* Implement support of CRL checking. OpenSSL 0.9.7 finally supports CRLs,
  so Postfix/TLS should support loading CRLs.

* Cleanup the "pfixtls" special logging, so that it fits Wietses original
  "per site" decision to make debugging easier.

* Move TLS based information from separate lines into Postfix's smtpd
  logging lines to make logfile analysis easier.

* Check the "info_callback" for sensitive use. I already had to remove the
  "warning alert" issued on normal shutdown. Why is a warning issued for
  a normal shutdown??

* Introduce new tls_per_client table to achieve the same selective behaviour
  for incoming connections.

* Introduce better support for "opportunistic" encryption: collect information
  about peers connecting; log warnings when the key changed etc.
  [I am not sure that I already have the best answers available.]

* Find a way to use the certificates themselves instead of the fingerprints
  to allow certificate based relaying. The maintenance of the fingerprints
  is a nightmare.