Fix crash while handling subshell trap (rhbz#1117404)

Contrary to the RH bug report, this is yet another bug with
virtual/non-forked subshells and has nothing to do with functions.
If a signal is ignored (empty trap) in the main shell while any
trap (empty or not) is set on the same signal in a subshell, a
crash eventually occurred upon restoring state when leaving the
subshell.

Original patch:
642af4d6/f/ksh-20120801-trapcom.patch

Prior discussion:
https://bugzilla.redhat.com/1117404

Paulo Andrade wrote there:
> The problem is that the sh_subshell function was saving pointers
> that could change, and when restoring, bad things would happen.
[...]
> The only comment I added:
> /* contents of shp->st.trapcom may change */
> may be a bit misleading, the "bad" save/restore already knows it,
> probably I should have added a better description telling that the
> data is, usually, modified in code like:
>
> tmp = buf[i]; buf[i] = strdup(tmp); free(tmp);
>
> so the shp->st.trapcom needs a "deep copy", as done in the
> patch, to properly save/restore pointers.

src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- sh_subshell(), sh_funscope(): Make *savsig/*savstak into a
  **savsig array. Use strdup(3) to save the data and get known
  pointers that will not change. Free these upon restore.
- Change the comment from the patch as Paulo wished he had done.

src/cmd/ksh93/tests/subshell.sh:
- Test 2500 times. This should trigger the crash most of the time.

src/cmd/ksh93/tests/subshell.sh

@@ -847,5 +847,19 @@ actual=${ get_value; }
 actual=`get_value`
 [[ $actual == "$expect" ]] || err_exit "\`Comsub\` failed to return output (expected '$expect', got '$actual')"
 
+# ======
+# https://bugzilla.redhat.com/1117404
+
+cat >$tmp/crash_rhbz1117404.ksh <<-'EOF'
+	trap "" HUP			# trigger part 1: signal ignored in main shell
+	i=0
+	for((i=0; i<2500; i++))
+	do	(trap ": foo" HUP)	# trigger part 2: any trap (empty or not) on same signal in subshell
+	done
+EOF
+got=$( { "$SHELL" "$tmp/crash_rhbz1117404.ksh"; } 2>&1)
+((!(e = $?))) || err_exit 'crash while handling subshell trap' \
+	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+
 # ======
 exit $((Errors<125?Errors:125))