zebra: collect and get netnamespaces information

zebra: collect and get netnamespaces information

upon zebra initialisation, and upon further netnamespace creation, the
the netnamespaces are created and a vrf associated to the netnamespace
is created. By convention, the name of the netns will be the same as the
VRF.
Add a stub routine that returns a fake ns identifier, in case netlink (
linux machines) is not available.
Also, upon each newly discovered NETNS, a NSID id being generated,
either by relying on kernel NSID feature, or by generating locally the
NSID ( see previous commit for more information).

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