`uintptr_t` maybe be easy to stick in a probe, but it is also a footgun.
This fixdes the display for e.g. port-process-return, but does not
affect the actual panics mentioned in #476.

It appears that JMP'd dtrace probes are being rewritten at load time
without a RTN. Near as I can tell, rustc offers no way to say 'don't
tail-call optimise this thing', so I'm forcing in a ZST with a drop impl
to achieve similar behaviour. Hopefully this suffices.

Realised when I was out that this struct only has 2B alignment, so there
was a possibility of passing up an unaligned reference to an SDT.

Given that we're now using these to handle nested `Ok(...)` types rather
than just error variants, a rename was a bit in order. `DError` might
also benefit?