Fix ioctl() and FIONREAD for sockets on Windows

This change fixes an issue where using FIONREAD would cause control flow
to jump to null, due to a _weaken() reference that I refactored long ago

test/posix/socket_fionread_test.c

@@ -0,0 +1,69 @@
+#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/ioctl.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;
+
+  // 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;
+
+  // accept client and send data
+  int server_client_fd;
+  if ((server_client_fd = accept(server_fd, 0, 0)) == -1)
+    return 8;
+  if (write(server_client_fd, "hi", 2) != 2)
+    return 9;
+
+  // poll to be safe (important for mac/bsd)
+  struct pollfd fds[] = {{client_fd, POLLIN}};
+  if (poll(fds, 1, -1u) != 1)
+    return 10;
+
+  // ask how many bytes we can read
+  uint32_t bytes_available;
+  if (ioctl(client_fd, FIONREAD, &bytes_available))
+    return 11;
+  if (bytes_available != 2) {
+    printf("wut %d\n", bytes_available);
+    return 12;
+  }
+
+  // clean up
+  if (close(client_fd))
+    return 13;
+  if (close(server_fd))
+    return 14;
+
+  CheckForMemoryLeaks();
+}