Simplify kdc_preauth.c systems table

Simplify kdc_preauth.c systems table

Get rid of static_preauth_systems, and replace it with explicit calls
to helper functions in get_preauth_hint_list() and return_padata().
Stop preallocating pa-data lists, instead reallocating on each
addition using add_pa_data_element().  Also simplify
maybe_add_etype_info2() using add_pa_data_element().

The KRB5_PADATA_PAC_REQUEST table entry did nothing, and was probably
originally added back when the KDC would error out on unrecognized
padata types.  The KRB5_PADATA_SERVER_REFERRAL entry has been disabled
since it was first added.