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/sock/recvfrom.c

@@ -21,6 +21,7 @@
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
 #include "libc/dce.h"
+#include "libc/intrin/describeflags.h"
 #include "libc/intrin/strace.h"
 #include "libc/nt/winsock.h"
 #include "libc/sock/internal.h"
@@ -95,7 +96,11 @@ ssize_t recvfrom(int fd, void *buf, size_t size, int flags,
   }
 
   END_CANCELATION_POINT;
-  DATATRACE("recvfrom(%d, [%#.*hhs%s], %'zu, %#x) → %'ld% lm", fd,
-            MAX(0, MIN(40, rc)), buf, rc > 40 ? "..." : "", size, flags, rc);
+  DATATRACE(
+      "recvfrom(%d, [%#.*hhs%s], %'zu, %s, %s) → %'ld% lm", fd,
+      MAX(0, MIN(40, rc)), buf, rc > 40 ? "..." : "", size, DescribeMsg(flags),
+      DescribeSockaddr(opt_out_srcaddr,
+                       opt_inout_srcaddrsize ? *opt_inout_srcaddrsize : 0),
+      rc);
   return rc;
 }