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)

examples/crashreport.c

@@ -22,7 +22,7 @@
  *     o//examples/crashreport.com
  */
 
-int main(int argc, char *argv[]) {
+noubsan int main(int argc, char *argv[]) {
   volatile int64_t x;
   ShowCrashReports();
   return 1 / (x = 0);