partial merge and full RCS ID sync with oksh

merged:
• new regression tests
• check.pl (tests/th) better tmpfile handling
• exec.c 1.50: POSIX specifies that for an AND/OR list,
  only the last command's exit status matters for "set -e"
• ksh.1 1.147: document the above
• eval.c 1.39: “Make $(< /nonexistent) have the same behaviour
  as $(cat /nonexistent) wrt. errors (do not unwind and do not
  treat this as fatal if set -e is used).”
  ‣ additionally make shf_open() return errno and actually show
    the error message from the system
• regression-39 test: remove the “maybe” marker
  ‣ but decide on correct POSIX behaviour

already been fixed in mksh:
• check.pl (tests/th) exit 1 if tests fail

not merged:
• main.c 1.52: run traps in unwind() before exiting;
  I’m pretty sure this is already working as-should in mksh
• eval.c 1.38: “Commands executed via `foo` or $( bar ) should
  not inherit "set -e" status.” As discussed in IRC, this is
  just plainly WRONG.

main.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: main.c,v 1.51 2012/09/10 01:25:30 tedu Exp $	*/
+/*	$OpenBSD: main.c,v 1.52 2013/06/15 17:25:19 millert Exp $	*/
 /*	$OpenBSD: tty.c,v 1.9 2006/03/14 22:08:01 deraadt Exp $	*/
 /*	$OpenBSD: io.c,v 1.22 2006/03/17 16:30:13 millert Exp $	*/
 /*	$OpenBSD: table.c,v 1.15 2012/02/19 07:52:30 otto Exp $	*/
@@ -34,7 +34,7 @@
 #include <locale.h>
 #endif
 
-__RCSID("$MirOS: src/bin/mksh/main.c,v 1.266 2013/06/03 22:28:04 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/main.c,v 1.267 2013/07/21 18:36:01 tg Exp $");
 
 extern char **environ;