rm "I/O error" error msg; just keep >0 exit status (re: 9011fa93)

The bug was really that I/O errors in output builtins were
undetectable by any means. Having a >0 exit status is sufficient.
Adding an error message risks making existing ksh scripts noisier,
or even breaking them if they redirect stderr to stdout.

Note to self: in future, implement the minimum change necessary to
fix bugs, nothing more. The fact that I needed to add four extra
2>/dev/null to the regression tests should have been a hint.

src/cmd/ksh93/bltins/print.c,
src/cmd/ksh93/data/msg.c,
src/cmd/ksh93/include/io.h:
- Remove "I/O error" message.

src/cmd/ksh93/tests/builtins.sh:
- Update to check for exit status only.

src/cmd/ksh93/tests/basic.sh,
src/cmd/ksh93/tests/coprocess.sh:
- Revert four new '2>/dev/null' to suppress the error message.

(cherry picked from commit 5e17be24d18455b575b6e98bc631c6935ffc795a)

NEWS

@@ -164,7 +164,7 @@ Any uppercase BUG_* names are modernish shell bug IDs.
   Ref.:	https://github.com/att/ast/issues/425
 	https://github.com/att/ast/pull/442
 
-- Fix BUG_PUTIOERR: Output builtins now correctly detect and report
+- Fix BUG_PUTIOERR: Output builtins now correctly detect
   input/output errors. This allows scripts to check for a nonzero exit
   status on the 'print', 'printf' and 'echo' builtins and prevent possible
   infinite loops if SIGPIPE is ignored.