Make the network_storage_* functions uniformly accept an enum value
instead of a string so that he conversion to string doesn't need to
happen in all callers.
network_settings_load expects NULL value to be returned
on failed attempts to read the settings files inside of
storage_network_open. At the same time storage_network_open
used to always return an initialized l_settings
structure despite the outcome of the read operations,
indicating a success.