x/sys/unix: add generic netlink types and constants on Linux

[mdlayher]

I'd like to add the types and constants found in <linux/genetlink.h>, as seen here: https://github.com/torvalds/linux/blob/master/include/uapi/linux/genetlink.h.

I will send a CL.

[gopherbot]

Change https://golang.org/cl/59931 mentions this issue: unix: add generic netlink types and constants on Linux