finalize server setup & close Unix socket if needed

[toastal]

Fixes #398

I’m not entirely sure what’s missing or if this is on my end. In my case I have been using dune exec bin/main.ml to launch where Dream.serve is called with Lwt_eio.run. I do not claim to be an expert in any sense on the topic.

[^1]

[^1]: Please consider giving up MS GitHub or offering a non-proprietary, non-US-corporate-controlled mirror for this free software project. I wish to delete this Microsoft account in the future, but I need more projects like this to support alternative methods to send patches & contribute.

[sabine]

• [ ] read up on man pages on sockets (POSIX)
• [ ] identify whether socket cleanup should be handled upstream in lwt (how do other ecosystems do it?), and whether/how lwt tells us to create a socket that is cleaned up reliably (see docs at https://ocsigen.org/lwt/5.5.0/api/Lwt_unix)

[aantron]

• [ ] Make sure the cleanup is done when the process is exited by signal.
• [ ] Document that Dream.serve does not do the cleanup.
• [ ] Don't try to unlink an abstract Unix domain socket.

[yawaramin]

After further discussion, it emerged that the common practice is to just optimistically delete the socket file before starting the server–if it doesn't exist then just carry on as normal.

@toastal are you able to change your PR to do this?