batman-adv: add netlink command to query generic mesh information files

BATADV_CMD_GET_MESH_INFO is used to query basic information about a
batman-adv softif (name, index and MAC address for both the softif and
the primary hardif; routing algorithm; batman-adv version).

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
[sven.eckelmann@open-mesh.com: Reduce the number of changes to
BATADV_CMD_GET_MESH_INFO, add missing kerneldoc, add policy for attributes]
Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>