UnixDatagram: please support vectored send

[joshtriplett]

The UnixDatagram type provides send and send_to methods, but does not provide a vectored send operation. Such an operation is particularly helpful for datagrams, where separate send operations result in separate datagrams; a vectored send allows sending data from multiple buffers in a single datagram.

Given that UnixDatagram only runs on UNIX, where writev works just fine on a socket, such an operation could use writev; alternatively, this could use sendmsg, which supports supplying an iovec. sendmsg would also allow a vectored send_to operation.

[sfackler]

We don't support vectored send/recv on UdpSocket either currently - PRs welcome for both!

[joshtriplett]

@sfackler Makes sense; UdpSocket filed separately as #68617.

[contrun]

I have just added vectored send for send in #69915 . It seems that we don't have binding for sendmsg from libc.

[Skgland]

I don't se it linked/mentioned here, but there are now unstable UnixDatagram::send_vectored_with_ancillary and UnixDatagram::send_vectored_with_ancillary_to tracked in #76915