net/failsafe: add fail-safe PMD

net/failsafe: add fail-safe PMD

Introduce the fail-safe poll mode driver initialization and enable its
build infrastructure.

This PMD allows for applications to benefit from true hot-plugging
support without having to implement it.

It intercepts and manages Ethernet device removal events issued by
slave PMDs and re-initializes them transparently when brought back.
It also allows defining a contingency to the removal of a device, by
designating a fail-over device that will take on transmitting operations
if the preferred device is removed.

Applications only see a fail-safe instance, without caring for
underlying activity ensuring their continued operations.

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>