Systemd socket activation

[reynir]

This PR is just to keep track.

Things to be done:

• [x] Fix socket permissions
• [ ] Sort out dependencies. The individual services depend on the other services sockets instead of their services. I'm not sure it should be done this way. It seems to Work For Me™ so far though.
• [x] Pure OCaml implementation of sd_listen_fds so we don't depend on the Linux-specific ocaml-systemd. Or some other way around this issue.

[hannesm]

this mostly looks fine - thanks for doing it.

I'm wondering that now the information about which socket name is used is in systemd (and the services are started accordingly). This also means that albatross_daemon that requires albatross_console.socket should take the socket name as argument, or not?

With this PR, the albatross_daemon will still use the hardcoded path from tmpdir/util/console.sock (as written in socket_path function in src/Vmm_core) -- does this matter (certainly not for a standard installation, but for someone who wants to modify the socket paths that makes a difference since they may think that they'd only need to adapt the systemd scripts)? Maybe a sentence in the Linux installation instructions is sufficient to not worry about that potential issue.

[hannesm]

thanks a lot, merged