PerlIO Layers: (Somewhat) better error messages

Hello p5p,

please find attached these two patches that I found laying around in my 
git repo. They change the somewhat uninformative error messages on IO 
layer mismatches to be a bit more specific.

I'm not sure whether my style of putting part of the error message into 
a sprintf parameter instead of directly into the (already long) string 
is acceptable.

All tests still pass, but I wonder how the t/porting/diag.t test is 
supposed to be run...

-max

• PerlIO Layers: (Somewhat) better error messages by Max Maischein