This patch simplifies the netmap backend code by means of the nm_open()
helper function provided by netmap_user.h, which hides the details of
open(), iotcl() and mmap() carried out on the netmap device.

Moreover, the semantic of nm_open() makes it possible to open special
netmap ports (e.g. pipes, monitors) and use special modes (e.g. host rings
only, single queue mode, exclusive access).
Signed-off-by: NVincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>