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.h

@@ -5,7 +5,23 @@
 #include <sys/types.h> /* ssize_t */
 #include <sys/uio.h> /* iovec */
 
-ssize_t send_fd(int dst_fd, int fd, const struct iovec *);
+
+/* send_fd(chanFd, fd, *iov)
+ *
+ * chanFd: fd to sendmsg over;
+ * fd: fd to send;
+ * iov: extra data to send or NULL;
+ *
+ * returns: result of sendmsg,
+ * i.e. the number of bytes sent */
+ssize_t send_fd(int chanFd, int fd, const struct iovec *);
+
+/* recv_fd(chanFd, flags)
+ *
+ * chanFd: fd to recvmsg from;
+ * flags: recvmsg flags e.g. 0, or MSG_CMSG_CLOEXEC?
+ *
+ * returns: the received fd or -1 */
+int recv_fd(int chanFd, int flags);
 
 #endif /* _CH_PROXY_SENFD */
-