zebra: add IPTABLE_ADD and IPTABLE_DEL commands in zapi

zebra: add IPTABLE_ADD and IPTABLE_DEL commands in zapi

Those messages permit a remote daemon to configure an iptable entry. A
structure is defined that maps to an iptable entry. More specifically,
this structure proposes to associate fwmark, and a table ID.
Adding to the configuration, the initialisation of iptables hash list is
done into zebra netnamespace. Also a hook for notifying the sender that
the iptables has been correctly set is done.

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