Port 465 with "implicit" (i.e. always-on) TLS is a more secure approach than port 587 with explicit (i.e. optional and only on with STARTTLS). Although we reject credentials on port 587 without STARTTLS, by that point credentials have already been sent.
All A/AAAA-resolvable domains that don't send or receive mail should have these null records.
This simplifies the handling of domains a bit by handling automatically generated subdomains more like other domains.
* Stop generating RSASHA1-NSEC3-SHA1 keys on new installs since it is no longer recommended, but preserve the key on existing installs so that we continue to sign zones with existing keys to retain the chain of trust with existing DS records.
* Start generating ECDSAP256SHA256 keys during setup, the current best practice (in addition to RSASHA256 which is also ok). See https://www.iana.org/assignments/dns-sec-alg-numbers/dns-sec-alg-numbers.xhtml#dns-sec-alg-numbers-1 and https://www.cloudflare.com/dns/dnssec/ecdsa-and-dnssec/.
* Sign zones using all available keys rather than choosing just one based on the TLD to enable rotation/migration to the new key and to give the user some options since not every registrar/TLD supports every algorithm.
* Allow a user to drop a key from signing specific domains using DOMAINS= in our key configuration file. Signing the zones with extraneous keys may increase the size of DNS responses, which isn't ideal, although I don't know if this is a problem in practice. (Although a user can delete the RSASHA1-NSEC3-SHA1 key file, the other keys will be re-generated on upgrade.)
* When generating zonefiles, add a hash of all of the DNSSEC signing keys so that when the keys change the zone is definitely regenerated and re-signed.
* In status checks, if DNSSEC is not active (or not valid), offer to use all of the keys that have been generated (for RSASHA1-NSEC3-SHA1 on existing installs, RSASHA256, and now ECDSAP256SHA256) with all digest types, since not all registers support everything, but list them in an order that guides users to the best practice.
* In status checks, if the deployed DS record doesn't use a ECDSAP256SHA256 key, prompt the user to update their DS record.
* In status checks, if multiple DS records are set, only fail if none are valid. If some use ECDSAP256SHA256 and some don't, remind the user to delete the DS records that don't.
* Don't fail if the DS record uses the SHA384 digest (by pre-generating a DS record with that digest type) but don't recommend it because it is not in the IANA mandatory list yet (https://www.iana.org/assignments/ds-rr-types/ds-rr-types.xhtml).
See #1953
This reverts commit b1d703a5e7 and adds python3-setuptools per the first version of #1899 which fixes an installation error for the b2sdk Python package.
