vale: change returning information of netmap_bdg_ctl with BDG_LIST
vale: change returning information of netmap_bdg_ctl with BDG_LIST

If nr_name contains a valid bridge name but not a port name,
the function returns that bridge index in nr_arg1 with
 NM_BDG_NOPORT in nr_arg2.
This feature is useful for an external module to see if a
specific bridge exists.
Currently OVS patch uses this feature.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap