Allow smtp_session_alloc() to fail gracefully and report an error.

With DANE TLSA lookups contributing to TLS policy, DNS problems make
errors much more likely, so we need to return a suitable DSN error
structure from smtp_tls_sess_alloc() via smtp_session_alloc().
Document smtp_tls_trust_anchor_file.
Reimplemented fingerprint support via DANE machinery and added per-site TAs.
Documentation of new DANE level and new parameters.

Hyperlink postconf.proto TLS security levels to TLS_README.html
Cosmetic TLS_README.html cleanup
Implemented and enabled DNS-based DANE security level.

    Question: Should lack of library support for the "dane" level
    (when chosen) just issue a warning or defer mail???
Rename smtp_tls_sess.c to smtp_tls_policy.c
New smtptls-finger utility.

This makes it much easier to test DANE, STARTTLS, etc, without
reconfiguring real mail flows.

Question: is the name smtptls-finger OK?
Question: is it OK to add a new file to $command_directory/ whose name
	  does not start with "post".
Question: Is it OK to install a mostly crippled version when TLS is
          disabled?  Without TLS all it can do is report the banner
	  and EHLO reply.

Note: The src/ sub-directory is the abbreviated name "tlsfinger",
but that seemed to generic a name to associate with an SMTP-only
utility.