lib: perform a bind inside vrf_socket() call

lib: perform a bind inside vrf_socket() call

This is an extension to previous behavior, where the bind() operation
was performed only when vrf was not a netns backend kind. This was done
like that because usually the bind parameter is the vrf name itself, and
having an interface name with vrf name is an expectation so that the
bind operation works.
the bind() operation can be performed on whatever device provided that
that name is not null and there is an interface in the vrf that has the
same name as the parameter.

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