node check: add --missing-slots check

This enables an explicit check for slots which should exist (according
to the repmgr metadata) but which aren't present.

doc/repmgr-node-check.sgml

@@ -77,6 +77,12 @@
       </simpara>
      </listitem>
 
+     <listitem>
+      <simpara>
+        <literal>--missing-slots</literal>: checks there are no missing replication slots
+      </simpara>
+     </listitem>
+
     </itemizedlist>
   </para>
   </refsect1>