Removing use of AF_NS and NS option

[Ryota Ozaki]

Hi,

There remain some stuffs of retired netns in the
source code. This attempt removes references to
AF_NS and NS option (and NETISR_NS definition).

Here is a patch:
http://www.netbsd.org/~ozaki-r/remove-netns.diff

Any objections to remove them?

  ozaki-r