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.
Fill in some missing postlink lmtp shadow parameters
Add ctable_refresh.

Applications that store TTLs in the cached values can request that
a stale entry to flushed and a new one fetched.
Polish docs for smtp_tls_trust_anchor_digests
Hyperlink postconf.proto TLS security levels to TLS_README.html
Cosmetic TLS_README.html cleanup
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,
changes that make use of this in later commits.

Related change to tls_dane.c to document tls_dane_avail() and make
it dependent on the required libresolv features.