cgtop: recursively count cgroup member tasks

When showing the number of tasks in a cgroup, recursively count tasks in
child cgroups and include them in the number. This ensures that the
number of tasks is cummulative the same way as memory, cpu and IO
resources are.

Old behaviour can be restored by passing the new --recursive=no switch.

man/systemd-cgtop.xml

@@ -176,6 +176,18 @@
         the count.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>--recursive=</option></term>
+
+        <listitem><para>Controls whether the number of tasks shown for
+        a control group shall include all tasks that are contained in
+        any of the child control groups as well. Takes a boolean
+        argument, defaults to <literal>yes</literal>. If enabled the
+        tasks in child control groups are included, if disabled only
+        the tasks in the control group itself are
+        counted.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>-n</option></term>
         <term><option>--iterations=</option></term>
@@ -212,7 +224,6 @@
 
   </refsect1>
 
-
   <refsect1>
     <title>Keys</title>