build: Make SystemdService entry conditional

When systemd user units installation is disabled, SystemdService
entry should also not be present on D-Bus service files.

This patch adds a common D-Bus service file template allowing,
an homogeneus D-Bus service file generation and also a
SystemService entry conditioned to the systemd user units
installation option.

https://bugzilla.gnome.org/show_bug.cgi?id=786149