Refactor KDC krb5_pa_data utility functions

Refactor KDC krb5_pa_data utility functions

Move alloc_padata from fast_util.c to kdc_util.c and make it
non-static so it can be used by other files.  Rename it to
alloc_pa_data for consistency with add_pa_data_element.  Make it
correctly handle zero length using a null contents pointer.

Make add_pa_data_element claim both the container and contents memory
from the caller, now that callers can use alloc_pa_data to simplify
allocation and copying.  Remove the copy parameter and the unused
context parameter, and put the list parameter first.  Adjust all
callers accordingly, making small simplifications to memory handling
where applicable.