sd-netlink: drop sd_genl_family_t and introduce GenericNetlinkFamily

Kernel manages each genl family by its name, e.g. "nlctrl" or WG_GENL_NAME,
and its ID (used for nlmsg_type) is determined dynamically when the
corresponding module is loaded.

This commit makes sd-netlink follow the same way; now, sd_genl_family_t
is dropped, and sd_genl_message_new() takes a genl family name. Each
genl family is resolved when it is used first time, and its information
is stored in GenericNetlinkFamily.

src/network/netdev/batadv.c

@@ -122,7 +122,7 @@ static int netdev_batadv_post_create(NetDev *netdev, Link *link, sd_netlink_mess
         b = BATADV(netdev);
         assert(b);
 
-        r = sd_genl_message_new(netdev->manager->genl, SD_GENL_BATADV, BATADV_CMD_SET_MESH, &message);
+        r = sd_genl_message_new(netdev->manager->genl, BATADV_NL_NAME, BATADV_CMD_SET_MESH, &message);
         if (r < 0)
                 return log_netdev_error_errno(netdev, r, "Failed to allocate generic netlink message: %m");