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.

NEWS

@@ -3,6 +3,13 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-12-29:
+
+- Fixed numerous build errors that prevented ksh from running on Haiku OS.
+
+- Added support for the SIGKILLTHR signal on operating systems that
+  implement it.
+
 2021-12-28:
 
 - Fixed a bug that caused CDPATH to continue working after unsetting it.