band-steering: add band-steering component

This adds a new band-steering component to usteer.

With band-steering enabled, usteer will attempt to move clients with a
consistently good SNR / signal from 2.4 GHz to the 5 GHz band.

In contrast to existing policies usteer tracks, band-steering is
non-invasive and will not lead to forceful connection termination.

Signed-off-by: David Bauer <mail@david-bauer.net>

node.h

@@ -59,6 +59,8 @@ struct usteer_local_node {
 
 	int beacon_interval;
 
+	uint16_t band_steering_interval;
+
 	struct {
 		bool present;
 		struct uloop_timeout update;