ripd: retrofit the 'passive-interface' command to the new northbound model

ripd: retrofit the 'passive-interface' command to the new northbound model

In ripd, the "passive-interface default" command has the following
behavior:
* All interfaces are converted to the passive mode;
* The "passive-interface IFNAME" command becomes a no-operation and
  "passive-interface IFNAME" statements are removed from the running
  configuration.
* The "no passive-interface IFNAME" can be used to remove interfaces
  from the passive mode.

This command was modeled using the following YANG data nodes in the
frr-ripd module:

  leaf passive-default {
    type boolean;
    default "false";
    description
      "Control whether interfaces are in the passive mode
       by default or not.";
  }
  leaf-list passive-interface {
    when "../passive-default = 'false'";
    type string {
      length "1..16";
    }
    description
      "A list of interfaces where the sending of RIP packets
       is disabled.";
  }
  leaf-list non-passive-interface {
    when "../passive-default = 'true'";
    type string {
      length "1..16";
    }
    description
      "A list of interfaces where the sending of RIP packets
       is enabled.";
  }

The 'when' statements guarantee that the list of passive interfaces
is cleared when the "passive-interface default" command is entered
(likewise, they guarantee that the list of non-passive interfaces is
cleared when the "passive-interface default" command is removed). This
matches exactly the behavior we want to model.

Finally, move the 'passive_default' global variable into the
'rip' structure where it belongs. This fixed the bug where the
"passive-interface default" command was being retained after a "no router
rip" + "router rip".

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>