New definition for managing tmpfiles.d(5) files.

This adds a new definition systemd::tmpfiles, for managing config
files in /etc/tmpfiles.d, for systemd-tmpfiles(8).

The API is a bit limiting, in that while you can give it multiple
paths to be written to the same tmpfiles.d config file, they will all
share the same type, mode, owner, et.c.  Something more flexible would
be nice, but I'll need to think about that more.  As it is, the API at
least works for a common situation, where you only want to manage a
single path (typically a directory) at a time.

And there are workarounds: you can either specify raw lines/chunks to
add to the config file using the 'content' parameter, or you can use
multiple resources (which would create one config file per resource,
but the semantics is the same).  It would be nice to have a slightly
higher level API for this, but this will do for now.