add an example systemd service file

The service file will run `/usr/bin/soju --config /etc/soju/config`.

By default it'll run as a dynamically created `soju` user and group (DynamicUser=yes),
and it will run in a constrained environment[1], only having write access to
/var/lib/soju and /run/soju

If the admin creates a static `soju` user and group, those static uid/gid will be used,
with the same service constraints. This can be useful to share the static `soju` group
with other services, so they can access the soju logs and database even
if the service is not running (or before it's running).

The procedure for the initial user and database scaffold still
works normally, when run as `root`:
```
mkdir /var/lib/soju/
sojudb -config /etc/soju/config create-user myname -admin
```
NOTE: systemd will automatically change the ownership of files in /var/lib/soju/ when the service
is started.

`sojuctl` will work for root, or with `sudo -u soju`, just normally.

References:
- [1] https://www.freedesktop.org/software/systemd/man/systemd.exec.html#DynamicUser=

contrib/soju.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=soju IRC bouncer service
+Documentation=https://soju.im/
+Documentation=man:soju(1) man:sojuctl(1)
+Wants=network-online.target
+After=network-online.target
+
+[Service]
+Type=simple
+User=soju
+Group=soju
+DynamicUser=yes
+StateDirectory=soju
+ConfigurationDirectory=soju
+RuntimeDirectory=soju
+ExecStart=/usr/bin/soju
+ExecReload=/bin/kill -HUP $MAINPID
+Restart=on-failure
+
+[Install]
+WantedBy=multi-user.target