DKIM support for multiple domains

[Daniel Graziotin]

As discussed in #83, SMF needs a way to handle DKIM for multiple domains.

In particular, it should gracefully handle the domain defined in $SMF_DOMAIN for backward compatibility (going back to single DKIM), create DKIM public/private keypairs to be used for all source domains in $SMF_CONFIG, adapt /var/db/dkim/ to a per-domain path style, and change /etc/opendkim/opendkim.conf to use KeyTable, SigningTable, ExternalIgnoreList, and InternalHosts.

Finally, SMF should check validity of all DKIM keys on startup.

This is underway.