second draft of I/O PDD

[Allison Randal]

I just committed a more complete draft of the I/O PDD to docs/pdds/ 
clip/pddXX_io.pod. I've integrated or responded to the mailing list  
comments. I also added a good bit of additional discussion of  
asynchronous operations and error handling. The original draft was  
"how it works now", while this draft is "how it should work", so a  
diff between the two provides an interesting comparison.

Comments and questions welcome as usual.
Allison