sysupdate: introduce "installdb" that keeps track of installed resources

Let's make sure we keep track of any file we drop into the system via a
database in /var/. This database is implemented based on symlinks, i.e.
reuses the fs as a simple database. Given the database most likely will
have <= 10 entries only (as we store *patterns* of installed file paths in
them, not the file paths themselves), this should be very efficient.

For implementation details see comments at top of
src/sysupdate/sysupdate-cleanup.c.

man/systemd-sysupdate.xml

@@ -204,6 +204,31 @@
         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>cleanup</option></term>
+
+        <listitem><para>Removes orphaned files that were previously installed by a transfer, but are no
+        longer owned by any currently defined transfer file. Whenever a resource is installed into the file
+        system, <command>systemd-sysupdate</command> records the target directory and the matching pattern in
+        an installation database below <filename>/var/lib/systemd/sysupdate/</filename>. This command
+        iterates through these records, determines which files they match, and deletes those that are no
+        longer covered by any of the patterns of the transfer files currently in place. This is useful to
+        garbage-collect files that used to be owned by a transfer file that has since been modified, disabled
+        or removed altogether (for example because a component is no longer being updated).</para>
+
+        <para>By default only the selected component is processed (i.e. the one selected via
+        <option>--component=</option>, or the default one if none were selected). Use
+        <option>--component-all</option> to process all components known to the installation database in a
+        single invocation.</para>
+
+        <para>This operation only removes files that were installed into the file system (i.e. resources of
+        type <literal>regular-file</literal>, <literal>directory</literal> and <literal>subvolume</literal>,
+        see <citerefentry><refentrytitle>sysupdate.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>);
+        it does not touch partition-based resources.</para>
+
+        <xi:include href="version-info.xml" xpointer="v262"/></listitem>
+      </varlistentry>
+
       <xi:include href="standard-options.xml" xpointer="help" />
       <xi:include href="standard-options.xml" xpointer="version" />
     </variablelist>
@@ -246,6 +271,19 @@
         <xi:include href="version-info.xml" xpointer="v251"/></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--component-all</option></term>
+        <term><option>-A</option></term>
+
+        <listitem><para>Instead of operating on a single component, operate on all known components (as well as
+        the default, component-less installation). This is currently only supported for the
+        <command>cleanup</command> command; all other commands will fail if this switch is used.</para>
+
+        <para>This option may not be combined with <option>--component=</option>.</para>
+
+        <xi:include href="version-info.xml" xpointer="v262"/></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--definitions=</option></term>