combining alarm and IFS caused segfault (rhbz#1176670)

The undocumented alarm builtin executes actions unsafely so that
'read' with an IFS assignment crashed when an alarm was triggered.

This applies an edited version of a Red Hat patch:
642af4d6/f/ksh-20120801-alarmifs.patch

Prior discussion:
https://bugzilla.redhat.com/1176670

src/cmd/ksh93/bltins/alarm.c:
- Add a TODO note based on dgk's 2014 email cited in the RH bug.
- When executing the trap function, save and restore the IFS table.

src/cmd/ksh93/sh/init.c: get_ifs():
- Remove now-unnecessary SHOPT_MULTIBYTE preprocessor directives as
  8477d2ce lets the compiler optimise out multibyte code if needed.
- Initialise the 0 position of the IFS table to S_EOF. This
  corresponds with the static state tables in data/lexstates.c.

src/cmd/ksh93/tests/builtins.sh:
- Crash test.

src/cmd/ksh93/tests/builtins.sh

@@ -1055,5 +1055,27 @@ do	case $bltin in
 			"(expected string containing $(printf %q "$expect"), got $(printf %q "$actual"))"
 done 3< <(builtin)
 
+# ======
+# The 'alarm' builtin could make 'read' crash due to IFS table corruption caused by unsafe asynchronous execution.
+# https://bugzilla.redhat.com/1176670
+if	(builtin alarm) 2>/dev/null
+then	got=$( { "$SHELL" -c '
+		builtin alarm
+		alarm -r alarm_handler +.001
+		i=0
+		function alarm_handler.alarm
+		{
+			let "(++i) > 100" && exit
+		}
+		while :; do
+			echo cargo,odds and ends,jetsam,junk,wreckage,castoffs,sea-drift
+		done | while IFS="," read arg1 arg2 arg3 arg4 junk; do
+			:
+		done
+	'; } 2>&1)
+	((!(e = $?))) || err_exit 'crash with alarm and IFS' \
+		"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+fi
+
 # ======
 exit $((Errors<125?Errors:125))