Updates to handle addition and removal of interfaces

[bjester]

Changes

• Converts some usage of assert that should likely persist if python -O is used
• Breaks out interface attachment from Zeroconf.__init__ into its own method _add_interfaces
• Adds corresponding behavior for removing interfaces, along the lines of how _add_interfaces functions but in reverse
• Exposes new public method Zeroconf.update_interfaces which triggers add or removal of interfaces compared with already attached interfaces
• Prevents stack trace reporting of errors that occur because the network is unreachable, errno.ENETUNREACH
• Updates Zeroconf.send to accept an interface on which filter the interfaces it sends the messages on, such that adding and removing interfaces can attempt to send add/remove messages when the interface is removed
  • Note: in Kolibri this will have little effect as we update the interfaces we detect the network has already changed, which will output warnings like WARNING: Network unreachable on interface 10.8.0.21
• Adds new integration test ensuring above behavior

Notes

• When the network is disconnected in Kolibri, it may take up to the TTL for events to occur on client and server
• With additional changes in my Kolibri PR to handle update_service events, I believe the following scenario is handled:
  • When the client is registered (connected to the VPN), drops out (disconnects from VPN), and rejoins with a new IP (reconnects with different VPN config) all within the TTL, the server may not have the correct information
• Kolibri PR: Sync resumption for SoUD syncs and cleanup handling kolibri#8183

[bjester]

It appeared that the OSError, socket.error exceptions had an explicit errno attribute so I made this match that, and also be defensive since the previous assert could be removed with python -O.

[bjester]

A fix to log socket write errors as simple warnings, as observed when interface becomes disconnected.

[rtibbles]

Tests pass, new test makes sense, new functionality is awesome!