bgpd: bgp support for netns

bgpd: bgp support for netns

The change contained in this commit does the following:
- discovery of vrf id from zebra daemon, and adaptation of bgp contexts
  with BGP.
  The list of network addresses contain a reference to the bgp context
  supporting the vrf.
  The bgp context contains a vrf pointer that gives information about
  the netns path in case the vrf is a netns path.

Only some contexts are impacted, namely socket creation, and retrieval
of local IP settings. ( this requires vrf identifier).

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