add new cmake option INSTALL_SYSTEMD_SERVICE_AND_INITSCRIPT requested for Gentoo packaging

fixes #6498

Signed-off-by: Gunnar Beutner <gunnar.beutner@netways.de>
Improve the systemd service definition file

- put "Icinga" back in the "Description=" field as usually only the description is displayed on unit start/stop or shown in syslog
- add dependency to carbon-cache.service
(systemd only cares about dependencies if the respective services are enabled, so this does not hurt if someone does not have carbon-cache
installed at all or is not using it at the moment)

fixes #6551

Signed-off-by: Gunnar Beutner <gunnar.beutner@netways.de>