Refactor KDC authdata list management helpers

Refactor KDC authdata list management helpers

Remove the unused concat_authorization_data().  Split merge_authdata()
into two helpers, one to destructively merge without filtering and one
to add copied elements while filtering out KDC-only authdata types.
Remove context parameters where they aren't needed (taking advantage
of knowledge that some libkrb5 functions don't use their context
parameters).