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/intrin/describemapflags.greg.c

@@ -25,6 +25,7 @@
 const char *DescribeMapFlags(int x) {
   _Alignas(char) static char mapflags[256];
   const struct DescribeFlags kMapFlags[] = {
+      {MAP_STACK, "STACK"},                      // order matters
       {MAP_ANONYMOUS, "ANONYMOUS"},              //
       {MAP_PRIVATE, "PRIVATE"},                  //
       {MAP_SHARED, "SHARED"},                    //
@@ -37,7 +38,6 @@ const char *DescribeMapFlags(int x) {
       {MAP_NORESERVE, "NORESERVE"},              //
       {MAP_NONBLOCK, "NONBLOCK"},                //
       {MAP_POPULATE, "POPULATE"},                //
-      {MAP_STACK, "STACK"},                      // order matters
   };
   return DescribeFlags(mapflags, sizeof(mapflags), kMapFlags,
                        ARRAYLEN(kMapFlags), "MAP_", x);