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.
* ospf_vty.c: (OSPF_VTY_GET_IF_PARAMS) new macro with common code for handling
  interface parameter commands - only used for 'ip ospf area' in this commit.
  (OSPF_VTY_PARAM_UNSET) similar
  ({no,}ip_ospf_area) Use said macros.
* doc/ospfd.texi: add 'ip ospf area' command.

Acked-by: Donald Sharp <sharpd@cumulusnetworks.com>
diff --git a/doc/ospfd.texi b/doc/ospfd.texi
index 856a2ba..7ddc9db 100644
--- a/doc/ospfd.texi
+++ b/doc/ospfd.texi
@@ -216,6 +216,7 @@
 @deffnx {OSPF Command} {network @var{a.b.c.d/m} area @var{<0-4294967295>}} {}
 @deffnx {OSPF Command} {no network @var{a.b.c.d/m} area @var{a.b.c.d}} {}
 @deffnx {OSPF Command} {no network @var{a.b.c.d/m} area @var{<0-4294967295>}} {}
+@anchor{OSPF network command}
 This command specifies the OSPF enabled interface(s).  If the interface has
 an address from range 192.168.1.0/24 then the command below enables ospf
 on this interface so router can provide network information to the other
@@ -239,6 +240,10 @@
 then we test whether the prefix in the network command contains
 the destination prefix.  Otherwise, we test whether the network command prefix
 contains the local address prefix of the interface. 
+
+In some cases it may be more convenient to enable OSPF on a per
+interface/subnet basis (@pxref{OSPF ip ospf area command}).
+
 @end deffn
 
 @node OSPF area
@@ -406,6 +411,19 @@
 @node OSPF interface
 @section OSPF interface
 
+@deffn {Interface Command} {ip ospf area @var{AREA} [@var{ADDR}]} {} 
+@deffnx {Interface Command} {no ip ospf area [@var{ADDR}]} {}
+@anchor{OSPF ip ospf area command}
+
+Enable OSPF on the interface, optionally restricted to just the IP address
+given by @var{ADDR}, putting it in the @var{AREA} area. Per interface area
+settings take precedence to network commands (@pxref{OSPF network command}).
+
+If you have a lot of interfaces, and/or a lot of subnets, then enabling OSPF
+via this command may result in a slight performance improvement.
+
+@end deffn
+
 @deffn {Interface Command} {ip ospf authentication-key @var{AUTH_KEY}} {}
 @deffnx {Interface Command} {no ip ospf authentication-key} {}
 Set OSPF authentication key to a simple password.  After setting @var{AUTH_KEY},