Allow ksh to compile on Haiku; implement SIGKILLTHR support (#408)

This commit implements the build fixes required to get ksh running on
Haiku. Note that while ksh does compile, it has a ton of regression test
failures on Haiku.

src/cmd/ksh93/data/signals.c,
src/lib/libast/features/signal.c:
- Add support for the SIGKILLTHR signal, which is supported by BeOS and
  Haiku.
- SIGINFO was missing an entry in the libast feature test, so add one
  (re: 658bba74).

src/cmd/ksh93/RELEASE:
- Add an entry noting that ksh now compiles on Haiku, albeit with many
  regression test failures.

src/cmd/ksh93/{include/terminal.h,sh/path.c}:
- Silence compiler warnings on Haiku.

src/lib/libast/features/mmap:
- The mmap feature test freezes on Haiku, so modify the test to fail
  immediately on that OS.

src/lib/libast/misc/signal.c:
- Avoid redefining the signal definition on Haiku to fix a compiler
  error.

src/lib/libast/features/nl_types:
- For some reason the nl_item typedef on Haiku doesn't work correctly.
  Work around that by creating the nl_item type in the libast nl_types
  feature test.

src/cmd/ksh93/sh/path.c

@@ -1417,6 +1417,7 @@ static noreturn void exscript(register char *path,register char *argv[],char **e
 	if(sh.sigflag[SIGCHLD]==SH_SIGOFF)
 		sh.sigflag[SIGCHLD] = SH_SIGFAULT;
 	siglongjmp(*sh.jmplist,SH_JMPSCRIPT);
+	UNREACHABLE();  /* silence warning on Haiku */
 }
 
 #if SHOPT_ACCT