ospfd: introduce ospf_lsa_minlen[] (BZ#705)

ospfd: introduce ospf_lsa_minlen[] (BZ#705)

This commit ports more packet checks to OSPFv2, in particular, LSA size
verification and Router-LSA link blocks verification.

* ospf_lsa.h: add LSA size macros
* ospf_packet.h: add struct ospf_ls_update
* ospf_packet.c
  * ospf_lsa_minlen[]: a direct equivalent of ospf6_lsa_minlen[]
  * ospf_router_lsa_links_examin(): new function, verifies trailing
    part of a Router-LSA
  * ospf_lsa_examin(): new function like ospf6_lsa_examin()
  * ospf_lsaseq_examin(): new function like ospf6_lsaseq_examin()
  * ospf_packet_examin(): add type-specific deeper level checks