Make improvements

- Get threads working on NetBSD
- Get threads working on OpenBSD
- Fix Emacs config for Emacs v28
- Improve --strace logging of sigset_t
- Improve --strace logging of struct stat
- Improve memory safety of DescribeThing functions
- Refactor auto stack allocation into LIBC_RUNTIME
- Introduce shell.com example which works on Windows
- Refactor __strace_thing into DescribeThing functions
- Document the CHECK macros and improve them in NDEBUG mode
- Rewrite MAP_STACK so it uses FreeBSD behavior across platforms
- Deprecate and discourage the use of MAP_GROWSDOWN (it's weird)

libc/calls/openat.c

@@ -23,6 +23,7 @@
 #include "libc/dce.h"
 #include "libc/fmt/magnumstrs.internal.h"
 #include "libc/intrin/asan.internal.h"
+#include "libc/intrin/describeflags.internal.h"
 #include "libc/log/log.h"
 #include "libc/str/str.h"
 #include "libc/sysv/consts/at.h"
@@ -75,7 +76,7 @@ int openat(int dirfd, const char *file, int flags, ...) {
   } else {
     rc = efault();
   }
-  STRACE("openat(%s, %#s, %s, %#o) → %d% m", __strace_dirfd(buf, dirfd), file,
+  STRACE("openat(%s, %#s, %s, %#o) → %d% m", DescribeDirfd(buf, dirfd), file,
          DescribeOpenFlags(flags), (flags & (O_CREAT | O_TMPFILE)) ? mode : 0,
          rc);
   return rc;