x/sys/unix: add taskstats.h types and constants on Linux

[mdlayher]

I'd like to add the types and constants found in http://elixir.free-electrons.com/linux/v4.8/source/include/uapi/linux/taskstats.h to x/sys/unix.

I will send a CL.

[gopherbot]

Change https://golang.org/cl/59050 mentions this issue: unix: add taskstats type and constants on Linux