new feature: DKIM message signing

I added a signing feature to my DKIM plugin.=20

https://github.com/qpsmtpd-dev/qpsmtpd-dev/blob/master/plugins/dkim

Matt

PS: for added pleasure, I also added a script that makes deploying DKIM =
really, really easy.  How easy?

	# cd ~smtpd/config/dkim
	# ./dkim_key_gen.sh example.org

Voila. Keys and selector generated. Now DNS needs to be updated. I made =
that easy too:

	# cat example.org/dns

apr2013._domainkey TXT =
"v=3DDKIM1;p=3DMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAt/Wu1fd74UXwH//=
0uiX/6C0hkv3I/PdeTxfnzHN6GrehJpCYBd1BKuigduwt/wZFVgUopwxmcjxSK6qrkADtHC+Zi=
qC/sqcVuVDhnvzkIgd7dYhqBcVORg6f8Eg8132yPkmHyDm588qKfdFSEUVgBqSfqZg4ZdG4Uq5=
erHAyQIEcs0h7xqUKJKA5xJWdRwaVYbNkNDAscax1WrSvMHQkKBf5bWUtkMGc/HeoZ6T3VTn5L=
e0OgLoINj4lNTFfT6toXsbZsKzOaUYacnWVOq2v2lWgghOMRQHYPr7ldl2E7/6sNSpNT8KXAiT=
7wlfE+/xXg+0DyQq/ahKaPgAecCCFiwIDAQAB"

Tell the world that the ONLY mail servers that send mail from this =
domain are DKIM signed and/or bear our MX and A records.

With SPF:

        SPF "v=3Dspf1 mx a -all"
        TXT "v=3Dspf1 mx a -all"

With DMARC:

_dmarc  TXT "v=3DDMARC1; p=3Dreject; adkim=3Ds; aspf=3Dr; =
rua=3Dmailto:[email protected]; =
ruf=3Dmailto:dmarc-feedback@'example.org; pct=3D100"

With DomainKeys (deprecated)

_domainkey TXT "o=3D-; t=3Dy; [email protected]"

For more information about DKIM and SPF policy, the documentation within =
each plugin contains a longer discussion and links to more detailed =
information:

   perldoc plugins/dkim
   perldoc plugins/sender_permitted_from