ScreenSaver: Revert the controversial parts of hrev46012

ScreenSaver once again locks up due to the bug described in #4260
awaiting a systemic fix.
BNetworkAddress: accept NULL link address strings.
net_server: Support joining networks by address.

* Until now, only the SSID name was being used to select a certain network.
* Now, you can also specify an address.
* Unfortunately, I could not test these changes, but they are part of ticket
  #10001.
Network: Fix associated networks, support store by MAC.

* The associated networks are now selected by address, not their name.
* Storing the settings now stores the address, too.
* This fixes #10001.