Refactor and unit-test all the code that looks at network interfaces to get current IP

Right now Tor has large hairy functions such as get_interface_address6() and get_interface_addresses_raw() that attempt to use system-specific API to try and get an IP address of network interface Tor instance is currently using. They should be refactored to improve code readability and maintainability. Also, we need to make sure there are unit tests for this kind of code.