[relax-ng] Implementation of restrictions on interleave

I implemented the proposed restrictions on interleave, including

- restrictions on element names for interleave not inside list
- restriction on <text/> for interleave not inside list
- restriction on <value/> for interleave not inside list
- disallowing list//interleave//data

It took me a morning to implement this.  However, I had thought about it on
the backburner for a few days before.

Overall it required quite a lot of thought to come up with an implementation
that was nice and simple without being terribly inefficient. I have a
unified implementation for both inside and outside of lists.  Overall the
inside of list case didn't make things much more complicated: there is about
50 lines specific to the inside of list case. On the other hand, I didn't
try to find a solution that handled just the outside of list case. By way of
comparison, this is a little but not much more than the checking for the
attribute names which look like namespace declarations.

I handled the <text/> case as part of my implementation of 7.2.

James