buffers: configurable buffer fault injector

buffers: configurable buffer fault injector

When configured at compile time via the cmake
VPP_BUFFER_FAULT_INJECTOR option, the buffer allocator will appear to
fail a certain fraction of the time.

By default, the allocator succeeds 80% of the time. Detailed command
line configuration options are available, but only when the image has
been compiled with cmake option described above:

    vlib { buffer-alloc-success-rate [0.0 ... 1.0]
           buffer-alloc-success-seed <nnnn> }

Modify vlib_buffer_pool_create(...) so 0 is always an invalid buffer
index.

Debug images: add checks for bad buffer index enqueues, and also
verify that f->n_vectors doesn't accidentally map one or more
instances of the frame poison pattern 0xfefefefe.

Type: improvement

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: Iab939858014463d1e664682805013d334d6fcbe5