zebra: socket operations stick to namespace if necessary

zebra: socket operations stick to namespace if necessary

Upon following calls: interface poll, address poll, route poll, and
ICMPv6 handling, each new Namespace is being parsed. For that, the
socket operations need to switch from one NS to one other, to get the
necessary information.

As of now, there is a crash when dumping interfaces, through show
running-config.

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