Add systemd unit files for downstream packagers (#612)

This commit stages some systemd unit files that could serve as a
reference for interested users or downstream Linux packagers.

There are two files: one to run the daemon itself, and one to set up
socket activation.

To enable roborev without socket activation you'd do:

    $ systemctl --user enable --now roborev.service

and to enable socket activation, you'd ensure `server_addr = "unix://"`
is set in the roborev config, and then

    $ systemctl --user disable --now roborev.service || true
    $ systemctl --user enable --now roborev.socket

Note that the `ExecStart` entry assumes the roborev binary will be
installed in `/usr/bin`, not `~/.local/bin`. I think this is correct for
downstream Linux packagers but it might be confusing for end users who
want to install these units "manually".

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>

Author: atheriel

packaging/systemd/roborev.service

@@ -0,0 +1,12 @@
+[Unit]
+Description=roborev code review daemon
+Documentation=https://github.com/roborev-dev/roborev
+
+[Service]
+Type=notify
+ExecStart=/usr/bin/roborev daemon run
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target

packaging/systemd/roborev.socket

@@ -0,0 +1,10 @@
+[Unit]
+Description=roborev daemon socket
+
+[Socket]
+ListenStream=%t/roborev/daemon.sock
+DirectoryMode=0700
+SocketMode=0600
+
+[Install]
+WantedBy=sockets.target