This routine fetches the next entry from id, returning
its values in *creds, and updates *cursor for
the next request. It requires that id identifies
a valid credentials cache and *cursor is
a cursor returned by krb5_cc_start_seq_get or
a subsequent call to krb5_cc_next_cred.
The krb5_end_seq_get routine is called
when no more entries are to be read.