This is needed to avoid issues with APs that restart their UPnP,
e.g., when ER reconfigures them. The previously known settings are
now cached and taken into use if an AP is detected to leave
(ssdp:byebye) and then return.
wps_er_start command now takes an optional parameter that can be used
to configure a filter to only allow UPnP SSDP messages from the
specified IP address. In practice, this limits the WPS ER operations
to a single AP and filters out all other devices in the network.
