service: use the same logic as systemd for appending .service to an unit

There's no need for complex rules to determine if `.service` should be
appended to a unit's name, just do like systemctl does.  For more
details see commit [696c245a23d55e4249651573eb9c61b68e61580c][1] and
[`unit_name_mangle`][2] from systemd.

[1]: http://cgit.freedesktop.org/systemd/systemd/commit/?id=696c245a23d55e4249651573eb9c61b68e61580c
[2]: http://cgit.freedesktop.org/systemd/systemd/tree/src/shared/unit-name.c?id=696c245a23d55e4249651573eb9c61b68e61580c#n501