Add MSG_ZEROCOPY from Linux 4.14 to bits/socket.h

[Florian Weimer]

On 11/15/2017 07:07 PM, Joseph Myers wrote:
> +    MSG_ZEROCOPY	= 0x4000000, /* Use user data in kernel path.  */
> +#define MSG_ZEROCOPY	MSG_ZEROCOPY

The constant value is the same as in the kernel header, so this loks okay.

Thanks,
Florian