The tlsconst internal package was created to provide human-readable
names for TLS versions and ciphers, from the IANA registry.
These days, that functionality is provided by stdlib's tls package, so
the internal package is no longer needed, and this patch removes it.
The only observable change is that the descriptive version in the
Received header now says "TLS 1.3" instead of "TLS-1.3", but it's just
as standard-compliant since that is in a free-form text section.
This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.
This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.
For now, it is experimental and the syntax and semantics are subject to
change.
Alberto Bertogli