Raw logs are stored in a versioned binary format and must be update with
atopconvert(1) upon atop version updates.
Failure to do so results in atop.service startup failure as I found out
the hard way after the "atop: 2.6.0 -> 2.7.1"[0] bump:
```
May 31 01:49:25 <hostname> sh[2269709]: existing file /var/log/atop/atop_20220531 has incompatible header
May 31 01:49:25 <hostname> sh[2269709]: (created by version 2.6 - current version 2.7)
May 31 01:49:25 <hostname> systemd[1]: atop.service: Main process exited, code=exited, status=7/NOTRUNNING
```
Convert logs in `ExecStartPre` and replace them iff updated.
This is to avoid changing original modification times upon every service
start and thus work against atop's log rotation (see existing
`ExecStartPre`).
0: https://github.com/NixOS/nixpkgs/pull/175180#issuecomment-1141546487
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
