nzbget: Fix script for copying default config file template (#51235)

* nzbget: Fix configFile / dataDir checking in service script

* nzbget: improve the description for the `configFile` option

* nzbget: Add detail to the `configFile` option description

* nzbget: Improve wording of `configFile` option

* nzbget: Refactor dataDir management into systemd config

* nzbget: Remove debug