add -U option to groupadd and groupmod

Add a -U option which adds new usernames as members.  For groupmod,
also add -a (append), without which existing members are removed.

Closes #265

man/groupadd.8.xml

@@ -229,6 +229,22 @@
 	  </para>
 	</listitem>
       </varlistentry>
+      <varlistentry>
+	<term>
+	  <option>-U</option>, <option>--users</option>
+	</term>
+	<listitem>
+	  <para>
+	    A list of usernames to add as members of the group.
+	  </para>
+	  <para>
+	    The default behavior (if the <option>-g</option>,
+	    <option>-N</option>, and <option>-U</option> options are not
+	    specified) is defined by the <option>USERGROUPS_ENAB</option>
+	    variable in <filename>/etc/login.defs</filename>.
+	  </para>
+	</listitem>
+      </varlistentry>
     </variablelist>
   </refsect1>