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/sh/init.c

@@ -503,21 +503,14 @@ static char* get_ifs(register Namval_t* np, Namfun_t *fp)
 		memset(shp->ifstable,0,(1<<CHAR_BIT));
 		if(cp=value)
 		{
-#if SHOPT_MULTIBYTE
-			while(n=mbsize(cp),c= *(unsigned char*)cp)
-#else
-			while(c= *(unsigned char*)cp++)
-#endif /* SHOPT_MULTIBYTE */
+			while(n = mbsize(cp), c = *(unsigned char*)cp++)
 			{
-#if SHOPT_MULTIBYTE
-				cp++;
 				if(n>1)
 				{
 					cp += (n-1);
 					shp->ifstable[c] = S_MBYTE;
 					continue;
 				}
-#endif /* SHOPT_MULTIBYTE */
 				n = S_DELIM;
 				if(c== *cp)
 					cp++;
@@ -533,6 +526,7 @@ static char* get_ifs(register Namval_t* np, Namfun_t *fp)
 			shp->ifstable[' '] = shp->ifstable['\t'] = S_SPACE;
 			shp->ifstable['\n'] = S_NL;
 		}
+		shp->ifstable[0] = S_EOF;
 	}
 	return(value);
 }