Linux service integration

[atavism]

For https://github.com/getlantern/engineering/issues/2836

• Adds Makefile targets to build/stage lanternsvc + systemd unit into the packaging root
• Adds Linux lanternsvc daemon entrypoint and wires FFI VPN controls to it over IPC

[garmr-ulfr]

@atavism heads-up, I am in the middle of making some changes to for authenticating the client connected to the socket and respect the provided socket path if it ends with the file name. ATM radiance will tack on radiance.sock to whatever is provided, assuming it's a directory.

[jigar-f]

@atavism Anything needed on my end here?

[atavism]

"I noticed you're making changes to the linux peer auth in radiance, but it should be accessible to anyone with sudo permissions, which is why I didn't require a user/group to be created for the lanternd.service in radiance. With the exception of using a daemon, lantern will work on linux using the same VPN calls as other platforms. I do need to make a small change (3 lines) to lantern, but other than that, the daemon should just need to be packaged. There shouldn't need to be any changes in radiance"

Can you check how this is working for you then, @garmr-ulfr? The PR has just been updated to package the daemon

[atavism]

With the exception of using a daemon, lantern will work on linux using the same VPN calls as other platforms

@garmr-ulfr So we don't need the FFI/IPC client changes here then? Can you remove those and please test the packaging updates?

[garmr-ulfr]

So we don't need the FFI/IPC client changes here then?

Nope. I tried to make it require as little changes to lantern as possible.

I should have a working version, including packaging, today.

[garmr-ulfr]

@atavism I didn't want to just push to your branch so I created a new one off yours. I opened a new PR(#8483) and defaulted to merge into main, but let me know if I should update it to merge into your instead!