DKIM check

[steffenfritz]

Describe the solution you'd like mxcheck should check for DKIM DNS entries.

We need a type for it and validation where possible.

• domain d=
• granularity g=
• acceptable algorithms h=
• key type k=
• note field n=
• public key p=
• selector s=
• testing t=
• version v=

See https://www.ietf.org/rfc/rfc6376.txt and updates in RFC 8301 and RFC 8463.