gpudev: support multi-process

gpudev: support multi-process

The device data shared between processes are moved in a struct
allocated in a shared memory (a new memzone for all GPUs).
The main struct rte_gpu references the shared memory
via the pointer mpshared.

The API function rte_gpu_attach() is added to attach a device
from the secondary process.
The function rte_gpu_allocate() can be used only by primary process.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>