Low: libcrmservice: Fix overrides for systemd resources

Previously, the override directory paths were valid only for systemd
resources whose type attribute has no extension and thus defaults to
".service". For example, 'class="systemd" type="httpd"' works, but
'class="systemd" type="httpd.service"' results in an override directory
/run/systemd/system/httpd.service.service.d. The same issue affected
resources for non-service units. For example, for a resource managing
unit "custom.timer", we would create an override directory
/run/systemd/system/custom.timer.service.d. In these cases, the
overrides would not be applied.

This commit fixes that. We also ensure that the [Service] section is
added only for service units.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>