systemctl: add commands set-default and get-default

systemctl set-default NAME links the default.target to the given unit,
get-default prints out the path to the currently set default target.

man/systemctl.xml

@@ -997,6 +997,24 @@ kobject-uevent 1 systemd-udevd-kernel.socket systemd-udevd.service
         </listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><command>get-default</command></term>
+
+        <listitem>
+          <para>Get the default target specified
+          via <filename>default.target</filename> link.</para>
+        </listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><command>set-default <replaceable>NAME</replaceable></command></term>
+
+        <listitem>
+          <para>Set the default target to boot into. Command links
+          <filename>default.target</filename> to the given unit.</para>
+        </listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><command>load <replaceable>NAME</replaceable>...</command></term>