Simplify krb5_dbe_def_search_enctype()

Simplify krb5_dbe_def_search_enctype()

Key data is now sorted in descending kvno order (since commit
44ad57d8d38efc944f64536354435f5b721c0ee0) and key enctypes can be
compared with a simple equality test (since single-DES support was
removed in commit fb2dada5eb89c4cd4e39dedd6dbb7dbd5e94f8b8).  Use
these assumptions to simplify krb5_dbe_def_search_enctype().

The rewrite contains one probably-unnoticeable bugfix: if enctype,
salttype, and kvno are all given as -1 in a repeated search, yield all
key entries of permitted enctype, not just entries of the maximum
kvno.