ses ioctl API/ABI stability

[Alan Somers]

We usually try to maintain backwards compatibility forever.  But is that
necessary for the ses(4) ioctls?  There are several problems with them as
currently defined.  They lack type safety, lack automatic copyin/copyout
handling, and one of them can overrun a user buffer.  I would like to fix
them, but adding backwards-compatibility versions would almost negate the
benefit.  Or, can we consider this to be an internal API, changeable at
will, as long as sesutil's CLI remains the same?
-Alan