Don't error on invalid enctypes in keytab

Don't error on invalid enctypes in keytab

krb5_ktfile_get_entry() used krb5_c_enctype_compare() to compare
enctypes, in order to share keys between single-DES enctypes.  As
key-sharing between enctypes is no longer done and single-DES support
has been removed, use a simple equality test to match the enctype.
This fixes a bug where krb5_kt_get_entry() would error out if the
keytab contained any entries with invalid enctypes (include single-DES
entries, after commit fb2dada5eb89c4cd4e39dedd6dbb7dbd5e94f8b8) even
if a matching entry is found.

[ghudson@mit.edu: rewrote commit message]

ticket: 8808