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/symlinkat.c

@@ -21,6 +21,7 @@
 #include "libc/calls/strace.internal.h"
 #include "libc/dce.h"
 #include "libc/intrin/asan.internal.h"
+#include "libc/intrin/describeflags.internal.h"
 #include "libc/sysv/consts/at.h"
 #include "libc/sysv/errfuns.h"
 
@@ -50,6 +51,6 @@ int symlinkat(const char *target, int newdirfd, const char *linkpath) {
     rc = sys_symlinkat_nt(target, newdirfd, linkpath);
   }
   STRACE("symlinkat(%#s, %s, %#s) → %d% m", target,
-         __strace_dirfd(buf, newdirfd), linkpath);
+         DescribeDirfd(buf, newdirfd), linkpath);
   return rc;
 }