systemctl,loginctl,machinectl: add --signal=list

This lists numerical signal values:
$ systemctl --signal list
SIGNAL NAME
1      SIGHUP
2      SIGINT
3      SIGQUIT
...
62     SIGRTMIN+28
63     SIGRTMIN+29
64     SIGRTMIN+30

This is useful when trying to kill e.g. systemd with a specific signal number
using kill. kill doesn't accept our fancy signal names like RTMIN+4, so one
would have to calculate that value somehow. Doing
  systemctl --signal list | grep -F RTMIN+4
is a nice way of doing that.

man/standard-options.xml

@@ -73,7 +73,8 @@
       <option>SIGTERM</option>.</para>
 
       <para>The special value <literal>help</literal> will list the known values and the program will exit
-      immediately.</para>
+      immediately, and the special value <literal>list</literal> will list known values along with the
+      numerical signal numbers and the program will exit immediately.</para>
     </listitem>
   </varlistentry>
 </variablelist>