shared/unit-file: make sure the old hashmaps and sets are freed upon replacement

shared/unit-file: make sure the old hashmaps and sets are freed upon replacement

Possibly fixes #15220. (There might be another leak. I'm still investigating.)

The leak would occur when the path cache was rebuilt. So in normal circumstances
it wouldn't be too bad, since usually the path cache is not rebuilt too often. But
the case in #15220, where new unit files are created in a loop and started, the leak
occurs once for each unit file:

$ for i in {1..300}; do cp ~/.config/systemd/user/test0001.service ~/.config/systemd/user/test$(printf %04d $i).service; systemctl --user start test$(printf %04d $i).service;done