Support for DNSSEC-validated lookups and TLSA RRsets.
Make parameter name reporting consistent in smtp+lmtp.

The smtp+lmtp delivery agent chooses its parameter personality (lmtp
or smtp) early in main() by looking at the tail of argv[0].

Later in smtp_state_alloc() it compared var_procname with "smtp"
and "lmtp" and again chose the same personality setting a flag in
the state structure.  This separate check creates an small opportunity
for inconsistent behaviour.  We make the original choice in main a
global variable, and make sure that var_procname in smtp_state_alloc()
is consistent with the initial choice.

The real motivation is however that in many cases the logging of
parameter names reported only the smtp(8) version of the parameter.
A couple of macros that test the new global "smtp_mode" made parameter
name logging consistent and much less tedious.
Replace disable_dns_lookups with backwards-compatible smtp_dns_support_level.

The new parameter can be empty (compatible) or else one of "disabled",
"enabled" or (if libresolv include files cooperate) "dnssec".

Plus documentation, and integration into the SMTP delivery agent:

    - When the level is "dnssec" enable DNSSEC lookups for MX records
      and any securely known hostnames.
    - When hostnames are securely validated, use the resolved hostname
      to name the SMTP session and to control TLS policy.
    - Pass the DNSSEC validation flag to smtp_session_alloc(), (not
      used yet).
Preliminary DANE support: per-destination trust-anchor digests.

Refactored internal verification callback machinery and re-implemented
fingerprint support via the new dane digest engine.

What remains now is to refactor smtp(8) to actually do DANE lookups
and build the appropriate policy structures on the fly.
New module for computing various digests and encoding fingerprints. Cut/paste from tls_dane.c.

Only real change is rename of tls_dane_serverid() to tls_serverid_digest().
Export functions to digest and fingerprint encode.

These will be needed to convert DANE digests from wire format.
Documentation cleanup.

Hyperlink postconf.proto TLS security levels to TLS_README.html
Cosmetic TLS_README.html cleanup
Split off session TLS policy parameters to a new structure.

We add a pointer to an SMTP_TLS_SESS to SMTP_SESSION to replace the
bunch of directly stored TLS parameters.

The SMTP_TLS_SESS structure is managed by a new module (which behaves
identically, after this commit, apart from the indirection).

This simplifies upcoming work.
Allow smtp_session_alloc() to fail gracefully and report an error.

With DANE TLS lookups contributing to TLS policy lookups DNS lookup
errors make this much more likely, so we need to return an suitable
DSN error structure from smtp_tls_sess_alloc() via smtp_session_alloc().
Add ctable_refresh.

Applications that store TTLs in the cached values can request that
a stale entry to flushed and a new one fetched.
Documentation of new DANE level and new parameters.
Implemented and enabled DANE security level.

    Question: Should lack of support for the chosen level just issue
    a warning or defer mail???

New utility function for clock-wrap safe time comparison.
Perhaps someone will still be using a 32-bit system in 2038.