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/describemapping.greg.c

@@ -0,0 +1,55 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2021 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/runtime/memtrack.internal.h"
+#include "libc/sysv/consts/map.h"
+#include "libc/sysv/consts/prot.h"
+
+static noasan char DescribeMapType(int flags) {
+  switch (flags & MAP_TYPE) {
+    case MAP_FILE:
+      return 'f';
+    case MAP_PRIVATE:
+      return 'p';
+    case MAP_SHARED:
+      return 's';
+    case MAP_STACK:
+      return 'S';
+    default:
+      return '?';
+  }
+}
+
+noasan char *DescribeProt(int prot, char p[hasatleast 4]) {
+  p[0] = (prot & PROT_READ) ? 'r' : '-';
+  p[1] = (prot & PROT_WRITE) ? 'w' : '-';
+  p[2] = (prot & PROT_EXEC) ? 'x' : '-';
+  p[3] = 0;
+  return p;
+}
+
+noasan char *DescribeMapping(int prot, int flags, char p[hasatleast 8]) {
+  /* asan runtime depends on this function */
+  DescribeProt(prot, p);
+  p[3] = DescribeMapType(flags);
+  p[4] = (flags & MAP_ANONYMOUS) ? 'a' : '-';
+  p[5] = (flags & MAP_GROWSDOWN) ? 'G' : '-';
+  p[6] = (flags & MAP_FIXED) ? 'F' : '-';
+  p[7] = 0;
+  return p;
+}