Add nl80211 commands to get and set o11s mesh networking parameters

The two new commands are NL80211_CMD_GET_MESH_PARAMS and
NL80211_CMD_SET_MESH_PARAMS. There is a new attribute enum,
NL80211_ATTR_MESH_PARAMS, which enumerates the various mesh configuration
parameters.

Moved struct mesh_config from mac80211/ieee80211_i.h to net/cfg80211.h.
nl80211_get_mesh_params and nl80211_set_mesh_params unpack the netlink messages
and ask the driver to get or set the configuration.  This is done via two new
function stubs, get_mesh_params and set_mesh_params, in struct cfg80211_ops.

Signed-off-by: Colin McCabe <colin@cozybit.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>