WIP: Add a systemd user service file

The build system still needs to be updated for this.

The unit should be installed as "mpDris2.service" in the directory shown
by `pkg-config systemd --variable=systemduserunitdir`.

(The default location is "/usr/lib/systemd/user/", but if systemd is not
available, then, IMHO, the unit file shouldn't be installed at all.)

Ref: issue eonpatapon#42
Add missing bus name to systemd unit

Ref: eonpatapon#42