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/kopenflags.S

@@ -0,0 +1,65 @@
+/*-*- mode:unix-assembly; indent-tabs-mode:t; tab-width:8; coding:utf-8     -*-│
+│vi: set et ft=asm ts=8 tw=8 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/fmt/magnumstrs.internal.h"
+#include "libc/macros.internal.h"
+
+	.macro	.e e s
+	.long	\e - kOpenFlags
+	.long	1f - kOpenFlags
+	.rodata.str1.1
+1:	.string	"\s"
+	.previous
+	.endm
+
+	.section .rodata
+	.align	4
+	.underrun
+kOpenFlags:
+	.e	O_RDWR,"RDWR"			// order matters
+	.e	O_RDONLY,"RDONLY"		//
+	.e	O_WRONLY,"WRONLY"		//
+	.e	O_ACCMODE,"ACCMODE"		// mask of prev three
+	.e	O_CREAT,"CREAT"			//
+	.e	O_EXCL,"EXCL"			//
+	.e	O_TRUNC,"TRUNC"			//
+	.e	O_CLOEXEC,"CLOEXEC"		//
+	.e	O_NONBLOCK,"NONBLOCK"		//
+	.e	O_DIRECT,"DIRECT"		// no-op on xnu/openbsd
+	.e	O_APPEND,"APPEND"		// weird on nt
+	.e	O_TMPFILE,"TMPFILE"		// linux, windows
+	.e	O_NOFOLLOW,"NOFOLLOW"		// unix
+	.e	O_SYNC,"SYNC"			// unix
+	.e	O_ASYNC,"ASYNC"			// unix
+	.e	O_NOCTTY,"NOCTTY"		// unix
+	.e	O_NOATIME,"NOATIME"		// linux
+	.e	O_EXEC,"EXEC"			// free/openbsd
+	.e	O_SEARCH,"SEARCH"		// free/netbsd
+	.e	O_DSYNC,"DSYNC"			// linux/xnu/open/netbsd
+	.e	O_RSYNC,"RSYNC"			// linux/open/netbsd
+	.e	O_PATH,"PATH"			// linux
+	.e	O_VERIFY,"VERIFY"		// freebsd
+	.e	O_SHLOCK,"SHLOCK"		// bsd
+	.e	O_EXLOCK,"EXLOCK"		// bsd
+	.e	O_RANDOM,"RANDOM"		// windows
+	.e	O_SEQUENTIAL,"SEQUENTIAL"	// windows
+	.e	O_COMPRESSED,"COMPRESSED"	// windows
+	.e	O_INDEXED,"INDEXED"		// windows
+	.long	MAGNUM_TERMINATOR
+	.endobj	kOpenFlags,globl,hidden
+	.overrun