[ietf-dkim] RFC 4871 errata

[Murray S. Kucherawy]

To answer the question about what current implementations do, I can speak 
for ours:

Regarding erratum 1383, we responded to the consensus and allow "*" 
anywhere, and more than once, in the "g=" value.

Regarding erratum 1378, we always include the "a=" in our generated 
signatures and require them when verifying, treating their absence as an 
error.  I would have no problem changing this behaviour to favour what 
section 3.3 says if that's consensus.