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/basic.sh

@@ -431,7 +431,7 @@ done | while read sec; do ( $sleep $sec; $sleep $sec) done
 s=SECONDS
 set -o pipefail
 for ((i=0; i < 30; i++))
-do	print hello
+do	print hello 2>/dev/null
 	sleep .1
 done |  $sleep 1
 (( (SECONDS-s) < 2 )) || err_exit 'early termination not causing broken pipe'
@@ -470,7 +470,7 @@ bintrue=$(whence -p true)
 set -o pipefail
 float start=$SECONDS end 
 for ((i=0; i < 2; i++))
-do	print foo
+do	print foo 2>/dev/null
 	sleep 1.5
 done | { read; $bintrue; end=$SECONDS ;}
 (( (SECONDS-start) < 1 )) && err_exit "pipefail not waiting for pipe to finish"