ripd: split-horizon default differed between rip_interface_new and _reset

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