New UDP definitions in Linux 3.16

[Joseph S. Myers]

Linux 3.16 adds constants UDP_NO_CHECK6_TX and UDP_NO_CHECK6_RX to 
uapi/linux/udp.h.  Do we want to add these to sysdeps/gnu/netinet/udp.h?  
(I suspect, notwithstanding the location of that file, some of the 
constants there are actually Linux-specific, maybe with the expectation 
that other OSes should use the same numbers if they adopt those features.)

-- 
Joseph S. Myers
joseph@codesourcery.com