taps: MVE

- TUNSETIFF works, the interface is indeed created.
- An fd is indeed passed over the unix socket,
- and is a valid (enough) fd because it can fed into `dup2`.
- `nix run -f . --offline pkgs.taps -- pass sleep 5` works, the
  interface exists for 5 seconds and disappears
- `nix run -f . --offline pkgs.taps -- pass ch-remote --api-socket=$HOME/uvms/nixos/vmm.sock add-net fd=3`
  obscurely fails, killing the VMM with:

  ```console
  [root@nixos:~]# cloud-hypervisor:  12.388270s: <_net1_qp0> ERROR:/build/source/net_util/src/queue_pair.rs:112 -- net: tx: failed writing to tap: Input/output error (os er
  ror 5)
  cloud-hypervisor:  12.388459s: <_net1_qp0> ERROR:virtio-devices/src/thread_helper.rs:54 -- Error running worker: HandleEvent(Error processing TX queue: NetQueuePair(Write
  Tap(Os { code: 5, kind: Uncategorized, message: "Input/output error" })))
  ```

pkgs/ch-proxy/sendfd.c

@@ -36,3 +36,39 @@ ssize_t send_fd(int dst_fd, int fd, const struct iovec *iov) {
 
     return (sendmsg(dst_fd, &msg, 0));
 }
+
+int recv_fd(int sock, int flags) {
+    int out = -1;
+
+    struct msghdr msg = { 0 };
+    struct cmsghdr *cmsg = NULL;
+    struct iovec iov = { 0 };
+    char dummy = 0;
+
+    msg.msg_iov = &iov;
+    msg.msg_iovlen = 1;
+
+    iov.iov_base = &dummy;
+    iov.iov_len = sizeof(dummy);
+
+    union {
+        struct cmsghdr align;
+        char buf[CMSG_SPACE(sizeof(int))];
+    } u;
+
+    msg.msg_control = u.buf;
+    msg.msg_controllen = sizeof(u.buf);
+
+    int bytes = 0;
+    if ((bytes = recvmsg(sock, &msg, flags)) < 0) {
+        perror("recv_fd: recvmsg");
+        return -1;
+    }
+    for (cmsg = CMSG_FIRSTHDR(&msg); cmsg != NULL; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
+        if (cmsg->cmsg_level != SOL_SOCKET) { continue; }
+        if (cmsg->cmsg_type != SCM_RIGHTS) { continue; }
+        if (CMSG_LEN(cmsg) < sizeof(out)) { continue; }
+        out = *(int*)CMSG_DATA(cmsg);
+    }
+    return out;
+}