Use EFF MTA-STS preload list

[foxcpp]

https://starttls-everywhere.org/policy-list/

Ensures a second reference point for TLS-enabled MTAs, compensating the MTA-STS downgrade trade-off (see RFC 8461 10.2).

• Embed initial list version in the code base
• Implement list auto-updating with (possibly) verification of the PGP signature
• Implement list lookups and other stuff
• Publish all that as a non-internal library so chasquid people can get on train together with us and not duplicate any effort

References

• https://github.com/EFForg/starttls-everywhere/blob/master/RULES.md

[foxcpp]

Factored out internal MTA-STS library into https://github.com/foxcpp/go-mtasts. EFF list support will be added there as a eff/ subpackage.

[foxcpp]

Implemented in c0a73bc.