x/sys/unix: add support for sending bulk udp packets

[hedwigz]

I wrote a custom file transfer application that uses udp over a one-way-connection (packets can only go one way from sender to receiver)
I used UdpConn.Write() for each packet. It works fine for 1Gbit connection but once you start
working on 10Gbit connections you can see that the syscall overhead starts to impact performance (you actually can't use the whole 10Gbit connection on some servers).
anyway there are apis for windows and linux to send more than one packet in one syscall.
I didn't dive deeply into linux's api but in windows I wrote a POC using the TransmtPackets function.
anyways I suggest adding this feature of sending multiple packets in one syscall, atleast for UDP at first.

[hedwigz]

here's an example code https://play.golang.org/p/kFQhC5v3kW

[bradfitz]

This is the sort of thing we would add to golang.org/x/sys instead of the core.