ospfd: introduce ospf_packet_minlen[] (BZ#705)

This commit ports some of the OSPFv3 packet reception checks
to OSPFv2.

* ospf_packet.c
  * ospf_packet_minlen[]: a direct equivalent of ospf6_packet_minlen[]
  * ospf_packet_examin(): new function designed after the first part
    of ospf6_packet_examin()
  * ospf_read(): verify received packet with ospf_packet_examin()
* ospf_packet.h: add convenience macros
diff --git a/ospfd/ospf_packet.h b/ospfd/ospf_packet.h
index 2115f11..3cbe889 100644
--- a/ospfd/ospf_packet.h
+++ b/ospfd/ospf_packet.h
@@ -46,6 +46,10 @@
 
 #define OSPF_HELLO_REPLY_DELAY          1
 
+/* Return values of functions involved in packet verification, see ospf6d. */
+#define MSG_OK    0
+#define MSG_NG    1
+
 struct ospf_packet
 {
   struct ospf_packet *next;