Bug#945269: debian-policy: packages should use tmpfiles.d(5) to create directories below /var

[Ansgar <ansgar@43-1.org>]

Package: debian-policy

I would like to recommend packages to use tmpfiles.d(5) to manage
creating directories in locations such as /var or /etc instead of
maintainer scripts.

It could also be used to create trivial files below /var that should be
recreated if deleted by accident (such as atd's sequence number which is
currently created by the maintainer script).

This has advantages over maintainer scripts:

 - It's declarative instead of imperative.

 - Directories/files get recreated automatically if deleted;
   previously packages would sometimes just fail if one deleted, for
   example, a subdirectory of /var/cache.

 - Manipulating ownership of files is prone to race conditions or fun
   with hard- and symlinks; many maintainer scripts don't do this fully
   correct.

 - It brings stateless systems (which start with empty /etc, /var) a bit
   closer.

I'm also fine with alternatives such as using `CacheDirectory=`,
`StateDirectory=`, `ConfigurationDirectory=` or `LogsDirectory=` in
.service files where appropriate.

I have no opinion on implementation details so far.

As far as I understand the current GR is unrelated to adopting things
like systemd-tmpfiles.

Ansgar