chasquid internal DKIM implementation

[Alberto Bertogli]

Hi!

I've been working on an internal DKIM implementation for chasquid.

While it should be considered experimental for now, it's in decent shape
and I've been trying it for a few days now, without issues.

If anyone is interested in giving it a try, you can find it in the
`next` branch (which is prone to being rebased):

https://blitiri.com.ar/git/r/chasquid/b/next/
https://github.com/albertito/chasquid/tree/next

The documentation for how to use it is already in the tree:

https://github.com/albertito/chasquid/blob/next/docs/dkim.md

Please let me know if you do, and if you have any feedback or
suggestions.

If you want to chat more interactively about it, #chasquid on oftc.net
is a convenient place for that :)

Thanks a lot!
Alberto