fpm: Add public header for Forwarding Plane Manager

The Forwarding Plane Manager (FPM) is an optional component that may
be used in scenarios where the router has a forwarding path that is
distinct from the kernel, commonly a hardware-based fast path. It is
responsible for programming forwarding information (such as routes and
nexthops) in the fast path.

In Quagga, the Routing Information Base is maintained in the 'zebra'
infrastructure daemon. Routing protocols communicate their best routes
to zebra, and zebra computes the best route across protocols for each
prefix. This latter information comprises the bulk of the Forwarding
Information Base.

The new header file added by this patch, 'fpm/fpm.h', defines a
point-to-point interface using which zebra can update the FPM about
changes in routes. The communication takes place over a stream
socket. The FPM listens on a well-known TCP port, and zebra initiates
the connection.

All messages sent over the connection start with a short 'FPM header'.
In the case of route add/delete messages, the header is followed by a
netlink message. Zebra should send a complete copy of the forwarding
table(s) to the FPM, including routes that it may have picked up from
the kernel.

The FPM interface uses replace semantics. That is, if a 'route add'
message for a prefix is followed by another 'route add' message, the
information in the second message is complete by itself, and replaces
the information sent in the first message.

If the connection to the FPM goes down for some reason, the client
(zebra) should send the FPM a complete copy of the forwarding table(s)
when it reconnects.

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

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