Add link to Tor SlackBuild on download-unix.html.en

I thought it would be nice to add link to the Tor SlackBuild on download-unix.html.en. The SlackBuild works fine and is updated regularly.

The code would be something like this:

<tr class="beige">
<td align="center"><img src="$(IMGROOT)/distros/slackware.png" alt="Slackware"></td>
<td>Slackware</td>
<td colspan="2"><a href="https://slackbuilds.org/repository/14.2/network/tor/">SlackBuilds.org</a></td>
<td>
<a href="<page docs/tor-doc-unix>">Linux/BSD/Unix</a><br>
</td>
</tr>

If nobody wants to design a new logo, there's generic.png in /images/distros/ folder