lib: add two APIs to handle socket operations with VRF NETNS

lib: add two APIs to handle socket operations with VRF NETNS

The vrf_sockunion_socket() wraps sockunion_socket() with vrf_id as
additional parameter. The creation of socket forces the user to
transparently move to new NETNS for doing the operation.
The vrf_getaddr_info() wraps getaddr_info() with vrf_id as additional
parameter. That API relies on the underlying system. Then there may be
need to switch to an other netns in that case too.
Also, the vrf_socket() implementation is simplified.

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