ripd: split-horizon default differed between rip_interface_new and _reset

* rip_interface.c: Default for split_horizon_default differed between
  rip_interface_new and rip_interface_reset, causing at least some issues
  after interface events. See patchwork #604. Fix, and consolidate code.

  (rip_interface_{reset,clean}) rename these to 'interface', as that's more
  appropriate.  Spin the ri specific bodies of these functions out to
  rip_interface_{reset,clean} helpers.  Factor out the overlaps, so
  rip_interface_reset uses rip_interface_clean.

  (rip_interface_new) just use rip_interface_reset.

* ripd.h: Update for (rip_interface_{reset,clean})

Reported by xufeng zhang, with a suggested fix on which this commit expands.
See patchwork #604.  This commit addresses only the split-horizon
discrepency, issue #2.  The other issue they reported, #1, is not addressed,
though suggested fix seems inappropriate.

Cc: xufeng.zhang@windriver.com
diff --git a/ripd/ripd.h b/ripd/ripd.h
index dbed342..5e87fcd 100644
--- a/ripd/ripd.h
+++ b/ripd/ripd.h
@@ -381,8 +381,8 @@
 extern void rip_reset (void);
 extern void rip_clean (void);
 extern void rip_clean_network (void);
-extern void rip_interface_clean (void);
-extern void rip_interface_reset (void);
+extern void rip_interfaces_clean (void);
+extern void rip_interfaces_reset (void);
 extern void rip_passive_nondefault_clean (void);
 extern void rip_if_init (void);
 extern void rip_if_down_all (void);