fpm: Add protobuf support for FPM.

fpm: Add protobuf support for FPM.

Code that allows a client to convey routes to a Forwarding Plane
Manager component using protobuf instead of netlink..

  * fpm/fpm.proto

    Protobuf definitions pertaining to the Forwarding Plane Manager.
    In particular, this file defines the AddRoute and DeleteRoute
    messages.

  * fpm/fpm.h

    Tweak FPM message header definition to also allow messages to be
    encoded in protobuf format.

  * fpm/{fpm_pb.h,.gitignore,.Makefile.am}

    Add the fpm_pb library, which contains code for interfacing with
    the FPM using protobuf.

  * configure.ac

    Generate fpm/Makefile.

  * Makefile.am

    Add fpm subdirectory to build.

  * common.am

    Add flags to be used by clients of the fpm_pb library.

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