alarm: don't save sh.ifstable (re: 18b3f4aa)

It is not correct to save sh.ifstable (a.k.a. shp->ifstable) before
calling a function and then restore it after; this can cause field
splitting to malfunction. See 70368c57.

The change to init.c in the Red Hat patch applied in 18b3f4aa
(shp->ifstable[0] = S_EOF) appears to be sufficient.

src/cmd/ksh93/bltins/alarm.c:
- Revert save/restore of sh.ifstable.

src/cmd/ksh93/tests/builtins.sh:
- Tweak the regression test to work correctly on a slower machine,
  i.e. a Raspberry Pi running FreeBSD 12.2 arm64 (thanks to hyenias
  for providing testing access).

src/cmd/ksh93/tests/builtins.sh

@@ -996,11 +996,11 @@ done 3< <(builtin)
 if	(builtin alarm) 2>/dev/null
 then	got=$( { "$SHELL" -c '
 		builtin alarm
-		alarm -r alarm_handler +.001
+		alarm -r alarm_handler +.005
 		i=0
 		function alarm_handler.alarm
 		{
-			let "(++i) > 100" && exit
+			let "(++i) > 20" && exit
 		}
 		while :; do
 			echo cargo,odds and ends,jetsam,junk,wreckage,castoffs,sea-drift