qpb: Add support for protobuf.

qpb: Add support for protobuf.

Infrastructure that allows protocol buffers to be used in Quagga. The
changes below comprise of:

  - Build hooks

  - Protobuf definitions for common types.

  - Library routines for working with protobuf, including functions
    that help translate between common quagga types and their protobuf
    equivalents.

Changes:

  * qpb/{Makefile.am,README.txt,qpb.h,.gitignore}

    Add the qpb library, which provides shared code and definitions
    for using protocol buffers in quagga code.

  * qpb/qpb.proto

    Protobuf definitions that can be shared by all of quagga.

  * qpb/linear_allocator.h

    An allocator that allocates memory by walking down towards the end
    of a buffer. This is used to cheaply allocate/deallocate memory on
    the stack for protobuf operations.

  * qpb/qpb_allocator.[ch]

    Thin layer that allows a linear allocator to be used with the
    protobuf-c library.

  * common.am

    This is an automake fragment that is intended to be shared by
    Makefile.am files in the tree. It currently includes definitions
    related to protobuf.

  * configure.ac

    - Add logic to optionally build protobuf code.

      By default, protobuf support is enabled if the protobuf C
      compiler (protoc-c) is available, and the associated header
      files/library can be found.

      The user can choose to override this behavior via the new
      --disable-protobuf/--enable-protobuf flags.

    - Include the quagga protobuf library (qpb) in the build.

  * .gitignore

    Ignore source code generated by protobuf compiler.

  * Makefile.am

    Add 'qpb' to the list of subdirectories.

Signed-off-by: Avneesh Sachdev <avneesh@sproute.com>
Edited: Paul Jakma <paul.jakma@hpe.com>: Change the sense of the
        configure enable option to require explicit specifying, as
        an experimental feature.