Refactor and test init_additional_STUN_servers.

* Make it accept any server sequence.
* Make it idempotent.
Test and refactor.

* Import modules, not names.
* Stricten port tests.
* Rename check_NAT_status to get_NAT_status.
* Test is_ipv6_address.
* Fix docstrings and test get_peer_url.
Merge pull request #841 from Renelvon/test_network_util

Mini refactor and test network util