This patch adds handling of a directory becoming a symlink in
/etc. Before this patch, the directory wasn't removed and then
symlinking failed, which caused directory not being updated at all.
The idea for the patch goes to @abbradar at
https://github.com/NixOS/nixpkgs/issues/16978#issuecomment-232921903:
> A heuristic idea for this -- a function `isStatic :: Path -> Bool`:
>
> * if path `/etc/foo` is a file, return True iff it's a symlink to `/etc/static/foo`.
> * if path is a directory, return True iff for all items in it `isStatic` is True.
>
> On any conflicts, if old path is static, it's safe to replace and/or
> delete stale. Otherwise make a backup and notify the user via a
> journal entry and console output.
The only difference here -- it will not replace user configs.
This also fixes https://github.com/NixOS/nixpkgs/issues/16978.
We now track copied files in /etc/.clean. This is important, because
otherwise files that are removed from environment.etc will not
actually be removed from the file system. In particular, changing
users.extraUsers.<user>.openssh.authorizedKeys.keys to an empty list
would not cause /etc/ssh/authorized_keys.d/<user> to be removed, which
was a security issue.
Some programs (notably the Java Runtime Environment) expect to be able
to extract the name of the time zone from the target of the
/etc/localtime symlink. That doesn't work if /etc/localtime is a
symlink to /etc/static/localtime. So make it a direct symlink.
