lib: split logicalrouter and vrf netns feature

lib: split logicalrouter and vrf netns feature

This split is introducing logicalrouter.[ch] as the file that contains
the vty commands to configure logical router feature. The split has as
consequence that the backend of logical router is linux_netns.c formerly
called ns.c. The same relationship exists between VRF and its backend
which may be linux_netns.c file.
The split is adapting ns and vrf fiels so as to :
- clarify header
- ensure that the daemon persepctive, the feature VRF or logical router
  is called instead of calling directly ns.
- this implies that VRF will call NS apis, as logical router does.

Also, like it is done for default NS and default VRF, the associated VRF
is enabled first, before NETNS is enabled, so that zvrf->zns pointer is
valid when NETNS discovery applies.

Also, other_netns.c file is a stub handler that will be used for non
linux systems. As NETNS feature is only used by Linux, some BSD systems
may want to use the same backend API to benefit from NETNS. This is what
that file has been done.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>