tmpfiles: use an ordered hashmap for the tmpfiles items

tmpfiles: use an ordered hashmap for the tmpfiles items

We should try to execute them in the same order they appear in the
configuration files, as it is documented. Hence move to an ordered
hashmap.

(Note though, that this still doesn't execute them completely in order:
we will still apply non-glob lines before glob-lines, and reorder lines
prefixing each other and that apply to the same paths).

http://lists.freedesktop.org/archives/systemd-devel/2015-March/029055.html