Fix a fork bomb when vi is run from a script and sent Ctrl-Z (#73)

This bug was reported on the old mailing list:
https://www.mail-archive.com/ast-developers@lists.research.att.com/msg00207.html

A fork bomb can occur when SIGTSTP is sent to the vi editor. Vi
must be launched from a script run with exec (tested with
BusyBox vi, nvi and vim):
$ cat /tmp/foo
vi /tmp/bar
echo end
$ ksh
$ chmod +x /tmp/foo
$ exec /tmp/foo
While in vi, send SIGTSTP using Ctrl-Z

src/cmd/ksh93/sh/fault.c:
- Only fork after Ctrl-Z if job control is available. The patch
  used checks 'job.jobcontrol' instead of 'SH_MONITOR':
  https://git.centos.org/rpms/ksh/blob/c8/f/SOURCES/ksh-20120801-forkbomb.patch

src/cmd/ksh93/sh/fault.c

@@ -519,7 +519,7 @@ void sh_exit(register int xno)
 	if(pp && pp->mode>1)
 		cursig = -1;
 #ifdef SIGTSTP
-	if(shp->trapnote&SH_SIGTSTP)
+	if((shp->trapnote&SH_SIGTSTP) && job.jobcontrol)
 	{
 		/* ^Z detected by the shell */
 		shp->trapnote = 0;