[PRD] ACTION-531 Update PRD examples complete

I first found it necessary to edit 2.5 Presentation Syntax to succinctly 
specify the EBNF for PRD PS.  I took a bit of
initiative and removed some of what I took to be arbitrary differences.  
E.g. I don't really like named argument UNITERMs, but I can see 
absolutely no reason why BLD should have them and PRD should not.

I had to reconcile 6.1 and 2.5.  I left Christian's extended Forall in 
6.1 but not in 2.5.

I then changed all the chicken examples (1.3, 2.3.1.2, 3.1) to use the 
PS defined in 2.5.

I think there are some minor inconsistencies between 2.5, 6.1, and 2.* 
where the XML syntax is explained.  I don't think I made it worse, 
though :-)