GUAC-748: Add username parameter to telnet.

guacamole/src/main/resources/net/sourceforge/guacamole/net/protocols/telnet.xml

@@ -3,6 +3,8 @@
     <param name="hostname" title="Hostname" type="text"/>
     <param name="port"     title="Port"     type="numeric"/>
 
+    <param name="username" title="Username" type="text"/>
+    
     <param name="font-name" title="Font name" type="text"/>
     <param name="font-size" title="Font size" type="enum">
         <option value=""></option>