zebra: add module to communicate routes to FPM

zebra: add module to communicate routes to FPM

Enhance zebra to send routes to the (optional) Forwarding Path Manager
component using the interface defined by fpm/fpm.h.

  * configure.ac

    - Add --enable-fpm flag.

      The FPM-related code in zebra is activated only if the build is
      configured with '--enable-fpm'.

    - Add HAVE_NETLINK automake conditional.

      This allows us to conditionally build netlink-dependent C code.

  * zebra/{rib.h,zebra_rib.c}

    - Add the 'fpm_q_entries' field to the rib_dest_t structure. This
      allows dests to be placed on the fpm queue.

    - Define a couple new rib_dest_t flags that hold FPM-related
      state.

    - Invoke the zfpm_trigger_update() function for a route_node
      whenever the information to be sent to the FPM changes.

    - rib_can_delete_dest(): Return FALSE if we have to update the FPM
      about the given dest. This ensures that the dest is not deleted
      even if there are no ribs hanging off of it.

  * zebra/zebra_fpm.c

    This file holds most of the code for interacting with the FPM.

    - If quagga was configured with '--enable-fpm', periodically try
      to connect to the FPM.

    - When the connection comes up, enqueue all relevent dests to the
      FPM queue.

    - When the FPM socket is readable, dequeue the next rib_dest_t
      from the FPM queue, encode it in to a message and send the
      message to the FPM.

    - When the connection to the FPM goes down, remove all dests from
      the FPM queue, and then start trying to connect to the FPM
      again.

    - Expose the following new operational commands:

      show zebra fpm stats
      clear zebra fpm stats

  * zebra/zebra_fpm_netlink.c

    - zfpm_netlink_encode_route(): Function to encode information
      about a rib_dest_t in netlink format.

  * zebra/zebra_fpm_private.h

    Private header file for the zebra FPM module.

  * zebra/zebra_fpm.h

    Header file exported by zebra FPM module to the rest of zebra.

  * zebra/debug.c

    Add the 'debug zebra fpm' command.

  * zebra/main.c

    Initialize the zebra-FPM code on startup.

  * zebra/misc_null.c

    Add stub for zfpm_trigger_update().

  * zebra/Makefile.am

    - Include new file zebra_fpm.c in build.

    - Include zebra_fpm_netlink.c in build if HAVE_NETLINK is defined.

  * vtysh/Makefile.am

    Include zebra_fpm.c in list of files that define cli commands.

Signed-off-by: Avneesh Sachdev <avneesh@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>