Cannot determine if device is active and in range

[whbruce]

Bluez caches devices found in previous discoveries so the lb_get_bl_devices() call can create a list with inactive entries. To speed up interrogation of devices it is useful to know which ones are active (i.e. powered on and and in range). This is possible by checking if the RSSI device property is greater than 0. However this property is only valid if discovery is in progress. To achieve this, we need to add an rssi field to device structure and three new API calls
lb_start_discovery()
lb_get_bl_devices_no_scan()
lb_stop_discovery()

Thanks to @petreeftime for this tip. If someone has a better idea, let me know.

I will implement for littleb.