Standardize spelling of "nonblocking"

Only adjusted the user-exposed messages and documentation,  not all
source code comments.
Fix typo in comment.

Fabrízio de Royes Mello
Don't try to pass -I switch to postmaster in contrib/start-scripts/linux.

Undo thinko in commit 87306184580c9c49717b00d48a2f9e717f21e0a8.
Per bug #8098 from Catherine Devlin.
Improve error message when an FDW doesn't support WHERE CURRENT OF.

If an FDW fails to take special measures with a CurrentOfExpr, we will
end up trying to execute it as an ordinary qual, which was being treated
as a purely internal failure condition.  Provide a more user-oriented
error message for such cases.