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)

NEWS

@@ -12,6 +12,11 @@ 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
+  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.
+
 2020-05-13:
 
 - Fix BUG_CASELIT: an undocumented 'case' pattern matching misbehaviour that