pkg/ns: refactored so that builds succeed on non-linux platforms

[aaithal]

Summary

moved functions that depend on linux packages (sys/unix) into a separate file
and added nop methods with a build tag for non-linux platforms in a new file.

Testing Done

./test ran successfully, apart from the macvlan Operations suite. I tested this on Amazon Linux and there's no support for macvlan on that platform. The failure log is pasted next:

• Failure [0.034 seconds]
macvlan Operations
/home/ec2-user/workspace/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/plugins/main/macvlan/macvlan_test.go:192
  creates an macvlan link in a non-default namespace [It]
  /home/ec2-user/workspace/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/plugins/main/macvlan/macvlan_test.go:102

  Expected error:
      <*errors.errorString | 0xc42014a690>: {
          s: "failed to create macvlan: operation not supported",
      }
      failed to create macvlan: operation not supported
  not to have occurred

  /home/ec2-user/workspace/src/github.com/containernetworking/cni/gopath/src/github.com/containernetworking/cni/plugins/main/macvlan/macvlan_test.go:87

[dcbw]

Should ns_linux.go have a +build linux in it?

[squeed]

Nah, ending a file with _$GOOS is an implicit build tag.

[squeed]

This makes me a bit uncomfortable: this is used in ns.Do(). I know that it currently won't cause problems, but...
Could you refactor Do() to use GetCurrentNS(), move GetCurrentNS() to the per-os files, and move getCurrentThreadNetNSPath to ns_linux.go?

[aaithal]

Yes, sure. That's cleaner than how I did it.

[aaithal]

@squeed I have updated the PR to address your comments, could you please take another look?

@dcbw as per @squeed's comments, the implicit tag should take care of it.

[squeed]

Wow, I'm surprised that netlink builds on non-linux :-).

The code changes LGTM. If anyone wants the commits to be squashed, that'd be reasonable.

[dcbw]

Updates LGTM.