#199 LDAP AUID selection improvement

[Marco Favero]

Hello,

with opendkim and ldap I can choose the auid (i=) from LDAP query:
* ldap://192.0.2.1/ou=people,dc=example,dc=com?DKIMSelector,DKIMIdentity?sub?(DKIMIdentity=$d)*

An instance from example in contrib folder is:

 dn: ou=bar,ou=people,dc=example,dc=com
objectClass: organizationalUnit
objectClass: top
objectClass: DKIM
DKIMSelector: bar-2011
DKIMIdentity: @bar.example.com

In many LDAP servers setup, it is common to base the mail addresses by account, and each account can share different domain aliases. For instance:

dn: ...
mail: me@example.com
mailalternateaddress: me@subdomain.example.com
mailalternateaddress: aliasme@subdomain.example.com
DKIMSelector: bar-2011

So it should be useful a "DKIMIdentity" multivalue and multiattribute, with opendkim that choose the value of auid corresponding to the From header of the mail:

• ldap://192.0.2.1/ou=people,dc=example,dc=com?DKIMSelector,mail,mailaternateaddress?sub?(|(mail=$d)(mailalternateaddress=$d))*

For instance, using above query If I sent a mail with header

From: me@example.com

I will see "i=me@example.com" in sign.

If I sent a mail with header

From: aliasme@subdomain@example.com

I will see "i=aliasme@subdomain@example.com" in header.