core: add {State,Cache,Log,Configuration}Directory= (#6384)

core: add {State,Cache,Log,Configuration}Directory= (#6384)

This introduces {State,Cache,Log,Configuration}Directory= those are
similar to RuntimeDirectory=. They create the directories under
/var/lib, /var/cache/, /var/log, or /etc, respectively, with the mode
specified in {State,Cache,Log,Configuration}DirectoryMode=.

This also fixes #6391.