For daemons which have a main configuration file, there's
little reason for the administrator to use configuration snippets.
They are useful for packagers which need to override settings, but
we shouldn't advertise that as the main way of configuring those
services.
https://bugs.freedesktop.org/show_bug.cgi?id=89397
Several manpages contain duplicate text describing a standard set of .d
configuration directories, with the usual sorting, precedence,
overrides, and so on. Factor this common text out using XInclude before
proliferating it even further.
