Fix: Closing a FD within a comsub broke output (rhbz#1116072)

Another Red Hat patch. "Prior to this update, the result of a
command substitution was lost if a file descriptor used for the
substitution was previously explicitly closed. With this update,
ksh no longer reuses file descriptors that were closed during the
execution of a command substitution. Now, command substitutions
work as expected in the described situation."

Prior discussion:
https://bugzilla.redhat.com/1116072

Original patch:
642af4d6/f/ksh-20140929-safefd.patch

src/cmd/ksh93/include/io.h,
src/cmd/ksh93/sh/io.c:
- Add sh_iosafefd() function to get a file descriptor that is not
  in use or otherwise occupied (including marked as closed).

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Use that function to obtain a safe FD upon restoring state when
  exiting a command substitution. I don't really know the how and
  why -- all that I/O magic is still beyond me and the code is
  uncommented as usual.

src/cmd/ksh93/tests/subshell.sh:
- Add regression test from the reproducer in the bug, reduced to
  the minimum necessary.

src/cmd/ksh93/tests/subshell.sh

@@ -830,5 +830,22 @@ sleep_pid=$!
 ((!(e = $?))) || err_exit "backtick comsub with pipe hangs (got status $e$( ((e>128)) && print -n / && kill -l "$e"))"
 kill "$sleep_pid" 2>/dev/null
 
+# ======
+# https://bugzilla.redhat.com/1116072
+
+expect='return-value'
+function get_value
+{
+	/dev/null/foo 2>/dev/null	# trigger part 1: external command (even nonexistent)
+	exec 3>&-			# trigger part 2: close file descriptor 3
+	print return-value		# with the bug, the comsub won't output this
+}
+actual=$(get_value)
+[[ $actual == "$expect" ]] || err_exit "\$(Comsub) failed to return output (expected '$expect', got '$actual')"
+actual=${ get_value; }
+[[ $actual == "$expect" ]] || err_exit "\${ Comsub; } failed to return output (expected '$expect', got '$actual')"
+actual=`get_value`
+[[ $actual == "$expect" ]] || err_exit "\`Comsub\` failed to return output (expected '$expect', got '$actual')"
+
 # ======
 exit $((Errors<125?Errors:125))