Support systemd socket activation for the local server

[heftig]

It would be nice if the local server (as launched by sccache --start-server) supported systemd socket activation, for both IP and UNIX sockets.

This would allow spawning sccache on demand and into its own cgroup, precisely controlling its lifetime, controlling its resources, giving systemd-oomd an isolated target to kill, and avoiding the hazards of forking, like ninja-build/ninja#2052.