Restore 'set -b'/'set -o notify' functionality (#74)

'set -b' had no effect; it should cause the shell to notify job
state changes immediately instead of waiting for the next prompt.

This fixes a regression that was introduced in ksh93t 2008-07-25.
The bugfix is from: https://github.com/att/ast/pull/1089

src/cmd/ksh93/sh/jobs.c:
- Save the tty wait state and avoid changing it if TTYWAIT was
  already on to avoid breaking 'set -b'.
  The last 'sh_offstate' is inside of an '#if' directive because it
  is only required when ksh is compiled with SHOPT_COSHELL enabled.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for 'set -b' in interactive shells.

src/cmd/ksh93/tests/pty.sh

@@ -549,5 +549,16 @@ r ^:test-2: true string\\r\\n$
 !
 done
 
+# err_exit #
+tst $LINENO <<"!"
+L notify job state changes
+
+# 'set -b' should immediately notify the user about job state changes.
+
+p :test-1:
+w set -b; sleep .01 &
+u Done
+!
+
 # ======
 exit $((Errors<125?Errors:125))