ospfd: Bring in some OSPF useful changes that were lost during Quagga fork

ospfd: Bring in some OSPF useful changes that were lost during Quagga fork

  Several changes were made from the original patch to resolve conflicts
  and also to fix various issues that were discovered during testing. Below
  is the original commit message minus a few parts that correspond to code
  that was dropped during bug fixing.

  Signed-off-by: Jafar Al-Gharaibeh <jafar@atcorp.com>
  ospfd: Extend 'ip ospf area' to take address argument + rationalise ospf enable
* ospfd.c: (general) Clean up the whole running of OSPF on interfaces.
  (add_ospf_interface) taking (struct interface *) arg is pointless here.
  (ospf_is_ready) new helper.
  (ospf_network_run_subnet) Put all the code for choosing whether to enable
  OSPF on a subnet, and if so which area configuration to use, here. If a
  subnet should not be enabled, ensure an existing oi is freed.
  (ospf_network_run_interface) Just call run_subnet for all subnets on an
  interface.
  (ospf_network_run) Just call run_interface for all interfaces.
  (ospf_if_update) Just call run_interface for the given interface.
  (ospf_network_unset) Just call run_subnet for existing ois.
  (ospf_update_interface_area) helper: update area on an oi, or create it.
  (ospf_interface_set) renamed to ospf_interface_area_set for clarity.
  Ensures OSPF is created, then into if_update.
  (ospf_interface_unset) renamed to ospf_interface_area_unset and collapses
  down to simple loop to call run_subnet for all ois.
* ospf_interface.h: add a more general OSPF_IF_PARAM_IS_SET, which does the
  right thing and takes default config into account.
* doc/ospfd.texi: add 'ip ospf area' command.

  Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
  This patch has been part of Quagga since October 2015

  Orignial Author:    Paul Jakma <paul@quagga.net>
  Date:      Thu Aug 27 16:51:42 2009 +0100