zebra: optionally use protobuf with FPM

zebra: optionally use protobuf with FPM

Change zebra so that it can optionally use protobuf serialization when
communicating with a Forwarding Plane Manager component.

  * zebra/main.c

    Add the --fpm-format/-F command line option. This allows the user
    to control the format (protbuf|netlink) that is used to
    communicate with the FPM.

  * zebra/zebra_fpm.c

    - zebra_init_msg_format(),

      This new function is invoked on process startup to determine the
      FPM format that should be used.

    - zfpm_init()

      Change to accept any 'FPM message format' specified by the user
      (via the new command line flag).

    - zebra_encode_route()

      Tweak to use the selected FPM format.

  * zebra_fpm_protobuf.c

    New code to build protobuf messages to be sent to the FPM.

  * zebra/Makefile.am

    - Include common.am

    - Build new file zebra_fpm_protobuf.c when protobuf is available.

    - Link with the fpm_pb library.

Signed-off-by: Avneesh Sachdev <avneesh@sproute.com>