nwfilter: enable filtering of gratuitous ARP packets

This patch enables filtering of gratuitous ARP packets using the following XML:

<rule action='accept' direction='in' priority='425'>
<arp gratuitous='true'/>
</rule>

docs/formatnwfilter.html.in

@@ -321,6 +321,7 @@
      <li>IPV6_ADDR: IPv6 address in numbers format, i.e., FFFF::1</li>
      <li>IPV6_MASK: IPv6 mask in numbers format (FFFF:FFFF:FC00::) or CIDR mask (0-128)</li>
      <li>STRING: A string</li>
+     <li>BOOLEAN: 'true', 'yes', '1' or 'false', 'no', '0'</li>
     </ul>
     <p>
      <br/><br/>
@@ -476,6 +477,11 @@
          <td>STRING</td>
          <td>text with max. 256 characters</td>
        </tr>
+       <tr>
+         <td>gratuitous <span class="since">(Since 0.9.2)</span></td>
+         <td>BOOLEAN</td>
+         <td>boolean indicating whether to check for gratuitous ARP packet</td>
+       </tr>
       </table>
     <p>
       Valid strings for the <code>Opcode</code> field are: