Callback-Id special case

[Nicolas Baradakis]

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