trivial-httpd: add option to specify the port

I use the trivial httpd server locally. Each time I restart the
server, I end up modifying manually the config file for other repos so
to point to the correct port. In this way I can just re-use the same
port.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

doc/ostree-trivial-httpd.xml

@@ -90,6 +90,14 @@ Boston, MA 02111-1307, USA.
                 </para></listitem>
             </varlistentry>
 
+            <varlistentry>
+                <term><option>--port</option>,<option>-P</option></term>
+
+                <listitem><para>
+                    Use the specified TCP port to listen on.
+                </para></listitem>
+            </varlistentry>
+
             <varlistentry>
                 <term><option>--force-range-requests</option></term>