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/log/showcrashreports.c

@@ -19,6 +19,8 @@
 #include "libc/calls/sigbits.h"
 #include "libc/calls/struct/sigaction.h"
 #include "libc/calls/struct/sigaltstack.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/log/backtrace.internal.h"
 #include "libc/log/internal.h"
 #include "libc/log/log.h"
 #include "libc/macros.internal.h"
@@ -33,6 +35,10 @@ STATIC_YOINK("__get_symbol_by_addr"); /* for asan memory origin */
 
 extern const unsigned char __oncrash_thunks[8][11];
 
+static void FreeSigaltstack(void *p) {
+  free(p);
+}
+
 /**
  * Installs crash signal handlers.
  *
@@ -67,7 +73,7 @@ void ShowCrashReports(void) {
   ss.ss_flags = 0;
   ss.ss_size = SIGSTKSZ;
   ss.ss_sp = malloc(SIGSTKSZ);
-  __cxa_atexit(free, ss.ss_sp, 0);
+  __cxa_atexit(FreeSigaltstack, ss.ss_sp, 0);
   sa.sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK;
   sigfillset(&sa.sa_mask);
   for (i = 0; i < ARRAYLEN(kCrashSigs); ++i) {