Until now network.c managed the list of network_info structs including
for known networks and networks that are seen in at least one device's
scan results, with the is_known flag to distinguish known networks.
Each time the list was processed though the code was either interested
in one subset of networks or the other. Split the list into a Known
Networks list and the list of other networks seen in scans. Move all
code related to Known Networks to knownnetworks.c, this simplifies
network.h. It also gets rid of network_info_get_known which actually
returned the list of all network_infos (not just for known networks),
which logically should have been private to network.c. Update device.c
and scan.c to use functions specific to Known Networks instead of
filtering the lists by the is_known flag.
This will also allow knownnetworks.c to export DBus objects and/or
properties for the Known Networks information because it now knows when
Known Networks are added, removed or modified by IWD.