Set if_index in update_device_info

This was previously set in setup_linklocal_addr and was lost during refactoring.
Rename check_ifaces to setup_ifaces and refactor it to simplify error handling
Rename HasFailed to IsReady and reverse logic
Move config_interface and kickoff_adverts calls into setup_ifaces
Don't reload the whole config when a netlink event is received, just re-setup the interfaces

The next step would be to make this finer-grained and setup only the interfaces
mentioned in the netlink messages.