Fix signal handling due to exit status > 256

This fixes two bugs: issuing the 'exit' command with a value > 256
would cause ksh 93u+ to kill itself with the corresponding signal
(try 'exit 265' to SIGKILL your interactive shell), and, if the
last command of a script exits due to a signal, the shell would
repeat that signal to itself, causing any parent ksh to also be
killed.

Discussion:
https://bugzilla.redhat.com/show_bug.cgi?id=1469624
https://rainbow.chard.org/2017/03/21/ksh-deliberately-segfaults-if-the-last-command-in-a-script-crashes/

This commit is loosely based on a patch applied to the 93v- beta
and the abandoned ksh2020, but that patch was incomplete & broken:
  $ ksh-2020.0.0 -c 'exit 265'; echo $?
  137
Expected: 9. Since the exit was *not* due to a signal, the value
should simply be cropped to the 8 bits supported by the OS.

src/cmd/ksh93/bltins/cflow.c: b_exit():
- For the 'exit' builtin command, bitwise-AND the argument to
  'exit' with SH_EXITMASK (8 bits, crop to 0-255) before passing it
  on to sh_exit(). This restores the behaviour of <=2011 ksh93
  versions and is in line with all other POSIX shells.
  It also fixes this bogosity:
    $ (exit 265); echo $?                   # non-forked subshell
    265
    $ (ulimit -t unlimited; exit 265); echo $?  # forked subshell
    9
  Forked or non-forked should make no difference at all
  (see commit message a0e0e29e for why).

src/cmd/ksh93/sh/fault.c: sh_done():
- If the current exit status is equal to the status for the last
  signal that was received from a child process, remove the
  SH_EXITSIG (9th) bit, so that the shell doesn't kill itself.
- If the shell's last child process exits due to a signal, exit
  with a portable 8-bit exit status (128 + signal number). This
  avoids the exit status being < 128 by being cropped to 8 bits.

src/cmd/ksh93/tests/signal.sh:
- Add regression test for exit with status > 256.
- Add regression test verifying the shell no longer kills itself.

(cherry picked from commit 98e0fc94393e175ce6adfee390327c320795bf12)

src/cmd/ksh93/sh/fault.c

@@ -643,7 +643,7 @@ void sh_done(void *ptr, register int sig)
 	sfsync((Sfio_t*)sfstdin);
 	sfsync((Sfio_t*)shp->outpool);
 	sfsync((Sfio_t*)sfstdout);
-	if(savxit&SH_EXITSIG)
+	if(savxit&SH_EXITSIG && (savxit&SH_EXITMASK) == shp->lastsig)
 		sig = savxit&SH_EXITMASK;
 	if(sig)
 	{
@@ -668,6 +668,11 @@ void sh_done(void *ptr, register int sig)
 	if(sh_isoption(SH_NOEXEC))
 		kiaclose((Lex_t*)shp->lex_context);
 #endif /* SHOPT_KIA */
+
+	/* Exit with portable 8-bit status (128 + signum) if last child process exits due to signal */
+	if (savxit & SH_EXITSIG)
+		savxit -= SH_EXITSIG + 128;
+
 	exit(savxit&SH_EXITMASK);
 }