tmpfiles: attach an array of items to each path

The data structure used by tmpfiles is changed: instead of hashmaps
mapping {path → Item*} we now have hashmaps containing
{path -> ItemArray}, where ItemArray contains a pointer
to an array of Items.

For current code it doesn't matter much, but when we add new types it
is easier to simply add a new Item for a given path, then to coalesce
multiple lines into one Item.

In the future, this change will also make it possible to remember the
file and line where each Item originates, and use that in reporting
errors. Currently this is not possible, since each Item can be created
from multiple lines.