Auto merge of rust-lang#69864 - LinkTed:master, r=Amanieu

unix: Extend UnixStream and UnixDatagram to send and receive file descriptors

Add the functions `recv_vectored_fds` and `send_vectored_fds` to `UnixDatagram` and `UnixStream`. With this functions `UnixDatagram` and `UnixStream` can send and receive file descriptors, by using `recvmsg` and `sendmsg` system call.

Author: bors

library/std/src/lib.rs

@@ -301,6 +301,7 @@
 #![feature(rustc_private)]
 #![feature(shrink_to)]
 #![feature(slice_concat_ext)]
+#![feature(slice_fill)]
 #![feature(slice_internals)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]