Added control of /sim/multiplay/hot to Multiplayer Settings dialogue.

This enables collisions with multiplayer aircraft.

defaults.xml

@@ -811,6 +811,7 @@ Started September 2000 by David Megginson, david@megginson.com
       <chat-menu include="ATC/chat-menu-entries.xml"/>
       <write-message-log type="bool" preserve="y">false</write-message-log>
       <freeze-on-replay type="bool" preserve="y">true</freeze-on-replay>
+      <hot type="bool" userarchive="y">false</hot>
       <default-model type="string" preserve="y">Models/Geometry/glider.ac</default-model>
       <callsign type="string" userarchive="y">callsign</callsign>
       <selected-server type="string" userarchive="y"/>

gui/dialogs/multiplayer.xml

@@ -181,6 +181,21 @@
             </binding>
         </checkbox>
 
+        <checkbox>
+            <row>3</row>
+            <col>1</col>
+            <colspan>2</colspan>
+            <halign>left</halign>
+            <name>hot</name>
+            <label>Enable collisions with other players</label>
+            <property>/sim/multiplay/hot</property>
+            <binding>
+                <command>dialog-apply</command>
+                <object-name>hot</object-name>
+            </binding>
+            <live>true</live>
+        </checkbox>
+
         <checkbox>
             <row>4</row>
             <col>1</col>