Add new Linux 3.16 constants to netinet/udp.h.

This patch adds the new constants UDP_NO_CHECK6_TX and
UDP_NO_CHECK6_RX from Linux 3.16 to sysdeps/gnu/netinet/udp.h.  (I
believe the existing constants there are already Linux-specific,
possibly with the intention that other OSes should adopt the same
values if possible if adopting the features in question.)

Tested on x86_64.

	* sysdeps/gnu/netinet/udp.h (UDP_NO_CHECK6_TX): New macro.
	(UDP_NO_CHECK6_RX): Likewise.

(cherry picked from commit 0bd72468)