Code Refactoring & Additional Tests

Thanks to the work by @BaxterStockman, the script has been refactored, hopefully making it easier to read and follow, while additional tests around IPv6 processing have been added.

Initial Release

First release of update-systemd-resolved. Should fully support the three standard DHCP options in OpenVPN (DNS, DOMAIN, and DOMAIN-SEARCH) with integration tests around the code to manage and monitor regressions. Also supports multiple (and combined) IPv4 and IPv6 DNS addresses.