Consistency in handling of self.interface
Consistency in handling of self.interface

switch_to_interface() becomes the common place where
self.interface is set; therefore self.interface is not None
precisely when self.default_server is connected.  Previously some
places required it to be connected and some didn't.  Also an
interface change now sends the 'updated' notification
consistently - previously some did and some didn't.

Have network_start() call start_interfaces() - this also means
network restarts now do this.

Fix apparent off-by-one in start_interfaces()

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