API CHANGE: Add a new "stopupdate" action for the sysadm/update class. (no additional inputs required).

This will look for any currently-running pc-updatemanager processes and kill/stop them as needed.

Example:
{
 "id":"dummy",
 "namespace":"sysadm",
 "name":"update",
 "args": {
    "action":"stopupdate"
 }
}

Output arguments:
"args":{
  "stopupdate":{
    "result":"success" or "error:<error text>"
  }
}

src/server/WebBackend.cpp

@@ -581,6 +581,10 @@ RestOutputStruct::ExitCode WebSocket::EvaluateSysadmUpdateRequest(const QJsonVal
       }else if(act=="startupdate"){
 	ok = true;
 	out->insert("startupdate", sysadm::Update::startUpdate(in_args.toObject()) );
+
+      }else if(act=="stopupdate"){
+	ok = true;
+	out->insert("stopupdate", sysadm::Update::stopUpdate() );
       }
 
     } //end of "action" key usage