Adding new option -rG to usermod

Adding a new switch -rG, which provides a similar feature set to
-aG, allowing a person to list exactly what groups to remove a
user from.

https://github.com/shadow-maint/shadow/issues/337

man/usermod.8.xml

@@ -326,6 +326,17 @@
 	  </para>
 	</listitem>
       </varlistentry>
+      <varlistentry>
+	<term>
+	  <option>-r</option>, <option>--remove</option>
+	</term>
+	<listitem>
+	  <para>
+	    Remove the user from named supplementary group(s). Use only with the
+	    <option>-G</option> option.
+	  </para>
+	</listitem>
+      </varlistentry>
       <varlistentry>
 	<term>
 	  <option>-R</option>, <option>--root</option>&nbsp;<replaceable>CHROOT_DIR</replaceable>