Check that afs3 salt is rejected for non-DES

The afs3 salt type is for compatibility with AFS-3 kaservers, which
are roughly krb4.  As such, it only makes sense for single-DES
enctypes.  The PBKDF2 and arcfour enctypes correctly reject the
key-creation parameters from the afs3 salt, but triple-DES currently
does not.