By convention /pkg is safe to use from outside the docker tree, for example
if you're building a docker orchestrator.
/nat currently doesn't have any dependencies outside of /pkg, so it seems
reasonable to move it there.
This rename was performed with:
```
gomvpkg -vcs_mv_cmd="git mv {{.Src}} {{.Dst}}" \
-from github.com/docker/docker/nat \
-to github.com/docker/docker/pkg/nat
```
Signed-off-by: Peter Waller <p@pwaller.net>
