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/describeopenflags.greg.c

@@ -0,0 +1,41 @@
+/*-*- 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 2022 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/fmt/itoa.h"
+#include "libc/fmt/magnumstrs.internal.h"
+#include "libc/intrin/describeflags.internal.h"
+#include "libc/mem/alloca.h"
+#include "libc/sysv/consts/sol.h"
+
+/**
+ * Describes clock_gettime() clock argument.
+ */
+char *DescribeOpenFlags(int x) {
+  char *s;
+  int i, n;
+  struct DescribeFlags *d;
+  _Alignas(char) static char openflags[128];
+  // TODO(jart): unify DescribeFlags and MagnumStr data structures
+  for (n = 0; kOpenFlags[n].x != MAGNUM_TERMINATOR;) ++n;
+  d = alloca(n * sizeof(struct DescribeFlags));
+  for (i = 0; i < n; ++i) {
+    d[i].flag = MAGNUM_NUMBER(kOpenFlags, i);
+    d[i].name = MAGNUM_STRING(kOpenFlags, i);
+  }
+  return DescribeFlags(openflags, sizeof(openflags), d, n, "O_", x);
+}