Rewrite Windows accept()

This change should fix the Windows issues Qt Creator has been having, by
ensuring accept() and accept4() work in O_NONBLOCK mode. I switched away
from AcceptEx() which is buggy, back to using WSAAccept(). This requires
making a tradeoff where we have to accept a busy loop. However it is low
latency in nature, just like our new and improved Windows poll() code. I
was furthermore able to eliminate a bunch of Windows-related test todos.

test/posix/accept_poll_test.c

@@ -0,0 +1,94 @@
+#include <arpa/inet.h>
+#include <cosmo.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <netinet/in.h>
+#include <poll.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int main() {
+
+  // Create server socket
+  int server_fd;
+  struct sockaddr_in address;
+  int addrlen = sizeof(address);
+  if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
+    return 1;
+  address.sin_family = AF_INET;
+  address.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+  address.sin_port = 0;  // let os assign random port
+  if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)))
+    return 2;
+  if (getsockname(server_fd, (struct sockaddr *)&address,
+                  (socklen_t *)&addrlen))
+    return 3;
+  if (listen(server_fd, SOMAXCONN))
+    return 4;
+
+  {
+    // poll server
+    struct pollfd fds[2] = {
+        {server_fd, POLLIN | POLLOUT},
+    };
+    int ret = poll(fds, 1, 0);
+    if (ret != 0)
+      return 5;
+  }
+
+  // create client socket
+  int client_fd;
+  if ((client_fd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
+    return 6;
+  if (connect(client_fd, (struct sockaddr *)&address, sizeof(address)))
+    return 7;
+
+  {
+    // poll server
+    struct pollfd fds[] = {{server_fd, POLLIN | POLLOUT}};
+    int ret = poll(fds, 1, -1u);
+    if (ret != 1)
+      return 8;
+    if (!(fds[0].revents & POLLIN))
+      return 9;
+    if (fds[0].revents & POLLOUT)
+      return 10;
+    if (fds[0].revents & POLLHUP)
+      return 11;
+    if (fds[0].revents & POLLERR)
+      return 12;
+  }
+
+  {
+    // poll server with invalid thing
+    struct pollfd fds[] = {
+        {server_fd, POLLIN | POLLOUT},
+        {666, POLLIN | POLLOUT},
+    };
+    int ret = poll(fds, 2, -1u);
+    if (ret != 2)
+      return 18;
+    if (!(fds[0].revents & POLLIN))
+      return 19;
+    if (fds[0].revents & POLLOUT)
+      return 20;
+    if (fds[1].revents & POLLIN)
+      return 21;
+    if (fds[1].revents & POLLOUT)
+      return 22;
+    if (!(fds[1].revents & POLLNVAL))
+      return 23;
+  }
+
+  // Clean up
+  if (close(client_fd))
+    return 13;
+  if (close(server_fd))
+    return 14;
+
+  CheckForMemoryLeaks();
+}