man: Factor out a common snippet for .d directories and precedence

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.

man/sysctl.d.xml

@@ -18,7 +18,8 @@
   You should have received a copy of the GNU Lesser General Public License
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 -->
-<refentry id="sysctl.d">
+<refentry id="sysctl.d"
+          xmlns:xi="http://www.w3.org/2001/XInclude">
 
         <refentryinfo>
                 <title>sysctl.d</title>
@@ -70,27 +71,6 @@
                 is <literal>#</literal> or <literal>;</literal> are
                 ignored.</para>
 
-                <para>Each configuration file shall be named in the
-                style of <filename><replaceable>program</replaceable>.conf</filename>.
-                Files in <filename>/etc/</filename> override files
-                with the same name in <filename>/usr/lib/</filename>
-                and <filename>/run/</filename>.  Files in
-                <filename>/run/</filename> override files with the same
-                name in <filename>/usr/lib/</filename>. Packages
-                should install their configuration files in
-                <filename>/usr/lib/</filename>. Files in
-                <filename>/etc/</filename> are reserved for the local
-                administrator, who may use this logic to override the
-                configuration files installed by vendor packages. All
-                configuration files are sorted by their filename in
-                lexicographic order, regardless of which of the
-                directories they reside in. If multiple files specify the
-                same variable name, the entry in the file with the
-                lexicographically latest name will be applied. It is
-                recommended to prefix all filenames with a two-digit
-                number and a dash, to simplify the ordering of the
-                files.</para>
-
                 <para>Note that either <literal>/</literal> or
                 <literal>.</literal> may be used as separators within
                 sysctl variable names. If the first separator is a
@@ -109,13 +89,6 @@
                 <filename>/proc/sys/net/ipv4/conf/enp3s0.200/forwarding</filename>.
                 </para>
 
-                <para>If the administrator wants to disable a
-                configuration file supplied by the vendor, the
-                recommended way is to place a symlink to
-                <filename>/dev/null</filename> in
-                <filename>/etc/sysctl.d/</filename> bearing the
-                same filename.</para>
-
                 <para>The settings configured with
                 <filename>sysctl.d</filename> files will be applied
                 early on boot. The network interface-specific options
@@ -142,6 +115,8 @@
                 example below).</para>
         </refsect1>
 
+        <xi:include href="standard-conf.xml" xpointer="confd" />
+
         <refsect1>
                 <title>Examples</title>
                 <example>