etc: add hnsd systemd service

[tuxcanfly]

WIP: looking for feedback to improve defaults, conflicts with other resolvers, tests on various linux distros etc.

[tuxcanfly]

So I don't think the config file is actually being parsed. Instead what I've done is pass the config options as command line params through the use of EnvironmentFile directive in systemd service.

[tynes]

Since hnsd.conf is no longer being parsed as a config file, should it be renamed to something like hnsd.env and have each of the possible environment variables in it but commented out? We would also need to add the environment variable parsing to hnsd for this to work.