Treat the transport option dkim_domain as a colon separated list, not

Treat the transport option dkim_domain as a colon separated list, not
as a single string, and sign the message with each element, omitting
multiple occurences of the same signer.

The transport option dkim_domain should be renamed to dkim_add_signers.
The values of dkim_selector and dkim_private_key are expanded for
each signer available in $dkim_domain.  It is unclear if signatures
for domains that already signed the mail should be omitted and if we
need a new variable for signatures to omit or if it could be hardcoded,
but this question is independent of this patch.