Make more Windows socket fixes and improvements

This change makes send() / sendto() always block on Windows. It's needed
because poll(POLLOUT) doesn't guarantee a socket is immediately writable
on Windows, and it caused rsync to fail because it made that assumption.
The only exception is when a SO_SNDTIMEO is specified which will EAGAIN.

Tests are added confirming MSG_WAITALL and MSG_NOSIGNAL work as expected
on all our supported OSes. Most of the platform-specific MSG_FOO magnums
have been deleted, with the exception of MSG_FASTOPEN. Your --strace log
will now show MSG_FOO flags as symbols rather than numbers.

I've also removed cv_wait_example_test because it's 0.3% flaky with Qemu
under system load since it depends on a process being readily scheduled.

libc/intrin/describemsg.c

@@ -0,0 +1,36 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8                               :vi │
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2024 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/intrin/describeflags.h"
+#include "libc/macros.h"
+#include "libc/sysv/consts/msg.h"
+
+const char *_DescribeMsg(char buf[16], int x) {
+  const struct DescribeFlags kMsgFlags[] = {
+      {MSG_FASTOPEN, "FASTOPEN"},    // order matters
+      {MSG_OOB, "OOB"},              //
+      {MSG_PEEK, "PEEK"},            //
+      {MSG_DONTROUTE, "DONTROUTE"},  //
+      {MSG_DONTWAIT, "DONTWAIT"},    //
+      {MSG_WAITALL, "WAITALL"},      //
+      {MSG_NOSIGNAL, "NOSIGNAL"},    //
+      {MSG_TRUNC, "TRUNC"},          //
+      {MSG_CTRUNC, "CTRUNC"},        //
+  };
+  return _DescribeFlags(buf, 16, kMsgFlags, ARRAYLEN(kMsgFlags), "MSG_", x);
+}