knownnetworks.c/.h implements the KnownNetworks interface and loads the
known networks from storage on startup. The list of all the networks
including information on whether a network is known is managed in
network.c to avoid having two separate lists of network_info structures
and keeping them in sync. That turns out to be difficult because the
network.c list is sorted by connected_time and connected_time changes
can be triggered in both network.c or knownnetworks.c. Both can also
trigger a network_info to be removed completely.
