x/sys/unix: add constants from linux/net_namespace.h

[squeed]

I'd like to add the NETNSA_ constants from net_namespace.h.

I want to use them in my PR for netlink[1] and I imagine they'd be used by other container libraries as well.

1: vishvananda/netlink#370,

[ianlancetaylor]

Go ahead. Thanks.

[gopherbot]

Change https://golang.org/cl/124436 mentions this issue: unix: add Linux network namespace constants