[krbdev.mit.edu #1231] kdc/kdc_util.c: add_to_transited_encoding fails to support null fields

[Sam Hartman via RT]

Among other things the function claims that it does not handle null
fields in an encoding which is required by RFC 1510.  

I also think the function seems rather complex  and seems to duplicate chunks of code a lot.  I believe it could benefit from abstraction.