Callback-Id special case

I'm going to remove the support for the deprecated 'Exec-Program' attribute, and while doing this I found in src/main/auth.c (around line 780) a special case for the 'Callback-Id' attribute, too. The RFC 2865 doesn't say anything unusual about 'Callback-Id', and I don't understand why it is processed in the 'authenticate' phase. I'm inclined to think it's an unnecessary hack, but I prefer to ask before doing a mistake. -- Nicolas Baradakis