UserAgent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36

Steps to reproduce the problem:
* Use a NaCl SDK build from a recent Chromium checkout (not sure which actual Pepper releases this affects, if any).
* Create a NaCl module that uses nacl_io.
* Create a UDP socket and use either recvmsg or sendmsg with msg->msg_iovlen > 1.
* Observe how the implementation in nacl_io will send (or receive) two or more UDP packets.

What is the expected behavior?
Both sendmsg and recvmsg should operate on a single message, as specified by POSIX.

https://linux.die.net/man/2/sendmsg
http://pubs.opengroup.org/onlinepubs/9699919799/functions/sendmsg.html

What went wrong?
The current recvmsg and sendmsg implementations (in src/native_client_sdk/src/libraries/nacl_io/kernel_proxy.cc, introduced in 710a20c325affb4c79d86680e2e365383e0f9759) will send/receive multiple messages if msg->msg_iovlen is greater than 1. However, a correct implementation should send only a single message and just interpret the msg->msg_iov array as a scattered buffer.

This matters for example if the underlying socket is a UDP socket.

Did this work before? N/A 

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