ospfd: Impl. per interface 'ip ospf area' command

Use with interface command:
 interface ppp0
 ip ospf area 0.0.0.0
This will enable OSPF on ppp0 with area 0.0.0.0

Remove with "no ip ospf area"

* ospf_vty.c: add "ip ospf area (A.B.C.D|<0-4294967295>)" interface command

* ospfd.c: (ospf_interface_{un,}set) new helper function to enable/disable
  OSPF on a specific interface.
  (ospf_if_update) 2 possible paths now to deal with interface updates.

Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/ospfd/ospf_interface.h b/ospfd/ospf_interface.h
index ed698c8..a437bfb 100644
--- a/ospfd/ospf_interface.h
+++ b/ospfd/ospf_interface.h
@@ -47,6 +47,7 @@
   DECLARE_IF_PARAM (u_int32_t, retransmit_interval); /* Retransmission Interval */
   DECLARE_IF_PARAM (u_char, passive_interface);      /* OSPF Interface is passive: no sending or receiving (no need to join multicast groups) */
   DECLARE_IF_PARAM (u_char, priority);               /* OSPF Interface priority */
+  DECLARE_IF_PARAM (struct in_addr, if_area);        /* Enable OSPF on this interface with area if_area */
   DECLARE_IF_PARAM (u_char, type);                   /* type of interface */
 #define OSPF_IF_ACTIVE                  0
 #define OSPF_IF_PASSIVE		        1