_kdc_find_etype: do not return success if ret_key != NULL
If _kdc_find_etype() is being called with 'ret_key' != NULL, the
caller is attempting to find an actual principal key. If 'ret_key'
is NULL then it is seeking a session key type. Only return an enctype
that is not in the principal key list unless 'ret_key' is NULL.
As part of this change remove 'clientbest' and the associated
logic as it is both unnecessary and can produce an enctype for
which the key cannot be returned.
Change-Id: Iba319e95fc1eac139f00b0cce20e1249482d2c6f