Fix BUG_PUTIOERR: Check for and report I/O error in output builtins

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.

sfsync() was already returning a negative value on I/O error, so
all we need to do is add a check. The stream buffer will need to be
filled before an I/O error can be detected, but this is the same on
other shells. See manual page: src/lib/libast/man/sfio.3

Ref.: https://github.com/att/ast/issues/1093
      https://github.com/att/ast/pull/1363

src/cmd/ksh93/bltins/print.c: b_print():
- Make sure an error result from sfsync() is reflected in the
  output builtin's exit status (exitval).
- Write an I/O error message (e_io) if the exit status is nonzero.

src/cmd/ksh93/data/msg.c, src/cmd/ksh93/include/io.h:
- Add the e_io[] error message.

src/cmd/ksh93/tests/builtins.sh:
- Add I/O error regression test, checking for the correct error
  message and exit status. All three output builtins use the same
  b_print() function so we only need to test one.

src/cmd/ksh93/tests/basic.sh,
src/cmd/ksh93/tests/coprocess.sh:
- Redirect stderr on a few 'print' commands to /dev/null; these
  now issue an expected I/O error. This does not cause failures.

NEWS, TODO:
- Update.

(cherry picked from commit 9011fa933552e483dab460f7dd1593d64e059d94)

src/cmd/ksh93/tests/builtins.sh

@@ -639,4 +639,26 @@ read baz <<< 'foo\\\\bar'
 : ~root
 [[ $(builtin) == *.sh.tilde* ]] &&  err_exit 'builtin contains .sh.tilde'
 
+# ======
+# Check that I/O errors are detected <https://github.com/att/ast/issues/1093>
+actual=$(
+    {
+        (
+            trap "" PIPE
+            for ((i = SECONDS + 1; SECONDS < i; )); do
+                print hi || {
+                    print $? >&2
+                    exit
+                }
+            done
+        ) | true
+    } 2>&1
+)
+expect=$': print: I/O error\n1'
+if [[ $actual != *"$expect" ]]
+then
+    err_exit "I/O error not detected (expected '$expect', got '$actual')"
+fi
+
+# ======
 exit $((Errors<125?Errors:125))